On 3 June 2015, a low weather system brought heavy rain to the coastal Otago Region particularly South Dunedin. South Dunedin experienced heavy flooding, which was exacerbated by the area's high water table and the breakdown of the Portobello pumping station. 1,200 homes and businesses were damaged by flood damage. The Dunedin City Council (DCC) described the 3 June 2015 flood event as a "one in a 100 year" flood, with about 175mm of rain falling during a 24-hour period. The DCC subsequently drew criticism for the poor state of the city's storm water infrastructure.
Background Geological history The South Dunedin plain was first formed about 18,000 years ago during the Last Glacial Period. At the time, the sea level was about 120 metres lower than today and the coastline stretched out as much as 35 km offshore further from its present extant. Following the Last Glacial Period, sea levels rose, reaching their present level about 7,000 years ago. The Otago Harbour was a former stream valley that was flooded during this period. During the post-glacial sea level rise, the South Dunedin plain was covered in ocean. A dune barrier was later formed between St Clair and Lawyers Head through the flow of fine sediment from the Clutha River and other smaller catchments. After the barrier formed, fine sediment accumulated in the waters at the head of the Otago Harbour, causing the sea in that area to became increasingly shallow. The gradual sediment accumulation led to the formation of a coastal wetland known as the South Dunedin plain, which also became a landbridge between the mainland South Island and the Otago Peninsula.
Human settlement and land reclaimation At the time of European settlement during the mid-19th century, the South Dunedin plain still consisted of salt marshes, wetlands and lagoons. The local vegetation consisted of tussock, rushes and flax. Local Māori people referred to this wetland system as Kaituna (which translates as "eating eels") due to the significant presence of eels in the area. A shallow lagoon also terminated through the salt dunes near contemporary St Clair. This flat coastal area functioned as a drainage basin for the hilly, catchment area that formed the rest of Dunedin. European settlers referred to this area as "The Flat." Since most of Dunedin is hilly, houses were initially built on the slopes of the extinct Dunedin Volcano. As the city's population grew during the Otago gold rush, European settlers began reclaiming swampy land in "The Flat." The land was filled using sand from the coastal dunes of St Kilda. Consequently, much of South Dunedin was built on land consisting of soft, sandy sediment that was only slightly above the water table (which was up to 17 cm lower than the present day).
During the late 19th century, marram grass was planted and sand-catching structures were also built along Ocean Beach, creating high sand dunes south of present-day Victoria Road that runs across St Kilda and St Clair. To accommodate the expansion of housing in South Dunedin, drainage of the wet lands through the dunes was blocked off to prevent storm tides from coming in through these dunes. During the 1960s and 1970s, further land reclamation occurred between Andersons Bay Road and Portsmouth Drive through the use of dredging spoil excavated from Otago Harbour. This reclaimed land was used predominantly for commercial activity and including a pumping station on Portobello Road. Land reclamation, human settlement, and the laying of asphalt and concrete for building roads and buildings in South Dunedin has disrupted the natural drainage basin by preventing water from seeping into the ground. In 2019, Stuff journalist Charlie Mitchell estimated that 60% of South Dunedin was impervious, with some pockets reporting 100%.
Natural disaster risks During the 1923 New Zealand Storm, heavy rain on 21 April led to extensive flooding in Caversham, New Zealand, South Dunedin, and St Kilda between 22 and 23 April. In addition, low-lying portions of the central and northern areas of Central Dunedin and North East Valley also experienced flooding in 1923. South Dunedin is also at risk of earthquake shaking and liquefaction due to several geographical faults including the Akatore Fault, which is situated between the Taieri Plains and Taieri Mouth. Due to its coastal location, South Dunedin is also vulnerable to sea level rise caused by climate change.
Socio-economic demographics South Dunedin has also historically experienced a high degree of socio-economic deprivation, with parts of the suburb ranking in the bottom 10 percent of the deprivation index. While the median personal income in South Dunedin is NZ$20,100, some pockets reported a record low median income of NZ$14,000. Due to its low-quality housing stock, South Dunedin also reported a high level of residents living in rental homes including low-income earners, people experiencing mental health and emotional issues, and recent migrants. Due to its flat landscape, South Dunedin also reported a significant number of residents using wheelchairs.
Flood event On 3 June 2015, the coastal parts of the Otago region in the South Island experienced heavy rainfall and high tide, which raised the level of height of the ground water and led to flooding. During the 24 hour period leading to 3pm on 3 June, Dunedin City Council (DCC) civil defence manager estimated that 90–95 mm of rain had fallen in the Dunedin area. The DCC described the June flood event as a "one in a 100 year flood," with about 175mm of rain falling in the 24 hour period between 4 am on 3 June and 4 am on 4 June, exceeding the "one in a 100 year flood" level which was 120mm over 24 hours.
The worst affected suburbs were South Dunedin, Kaikorai Valley, the Brighton coast, the Taieri Plain, North Road and parts of Mosgiel. In response to the rainfall and flooding, residents erected sandbags to protect their properties and businesses. St Clair and Bathgate Park primary schools also sent their pupils home in response to the flooding.
By 12:40 pm, Pine Hill had recorded over 60mm of rain in the past 24 hours. Heavy rain also caused several rivers and streams in North Otago, Dunedin, South Otago and upper Taieri Plain including the Silver Stream, Kaikorai Stream, the Water of Leith, and Lindsay Creek to rise rapidly. Flooding was reported in Kaikorai Valley road, the Gordon Road spillway, and parts of the Taieri Plain and North East Valley. In addition, heavy rainfall caused leaks at several Dunedin Hospital properties, with a blocked city main causing flooding at the Hospital's Lower Ground loading dock.
By noon, MetService duty forecaster Emma Blake reported that 70mm of rain had fallen in coastal Otago during the morning. The meteorological service maintained a heavy rain warning for Dunedin and the Clutha District until 2am on 4 June but forecast that rain would slowly begin easing overnight. Blake forecast that Dunedin would receive a further 80 to 100 mm of rainfall.
The rain eased on 4 June, with Dunedin receiving a total of 175 mm of rain over the past 24 hours. South Dunedin was the hardest hit area, with Labour Member of Parliament for Dunedin South, Claire Curran, describing the suburb as a "major disaster area."
Impact Dunedin The heavy rainfall and flooding strained Dunedin's stormwater and sewage systems and road network. By 10:30 am on 3 June, the DCC Water and Waste Network Contracts Manager Mike Ind confirmed that stormwater and sewers in the Hillside Road and Surrey Street areas had reached capacity. By 4pm on 3 February, nine roads around the wider Dunedin area had been closed. New Zealand Transport Agency (NZTA) urged people to avoid traveling on roads and highways around Dunedin. The NZTA also advised people to avoid travelling on the Dunedin Southern Motorway between Dunedin city centre and Mosgiel. In addition, foul sewer contamination had led to the closure of Hargest Crescent road area. Radio New Zealand later revised the number of affected roads to 15.
By 3 June, Radius Fulton rest home on Dunedin's Hillside Road had been evacuated due to flooding. While some of Radius Fulton's 94 residents managed to seek shelter with family, the rest home had to arrange alternative accommodation for 78 residents. Multiple slips were also reported in the Otago peninsula including Taiaroa Head, which obstructed travel in the area. Two drivers also escaped after their cars were swallowed up by a sinkhole in the Otago peninsula.
East Taieri In response to surface flooding, power utilities company Aurora Energy cut power to 160 homes in the East Taieri area until the floodwaters subside and power could be safely restored, affecting 517 consumers. By 4 June, Auroa Energy had restored power to the affected homes.
South Otago Surface flooding was also reported in the Clutha District, leading to the closure of several roads including Lakeside Road at the railway underbridge, Akatore Road at Big Creek, Papatowai Highway at Caberfeidh Hill, Karoro Creek, Mt Wallace Road, Back Road, Springfield Road, Allison Road, and Remote Road.
Central Otago Flooding also occurred on parts of State Highway 8 between the Lindis Pass and Cromwell in Central Otago.
Tasman-Nelson region Over 50mm of rain fell in the Tasman–Nelson region of the upper South Island between the mornings of 2 and 3 June 2015, resulting in properties and paddocks being flooded. In addition, Cable Bay near Nelson experienced heavy rain and flooding.
Responses Emergency response In response to the flooding, emergency services established an operations centre at the civil defence bunker in Dunedin Central to coordinate their response to the various flooding events across Dunedin. On 4 June, Fire Service personnel also commenced pumping activities throughout South Dunedin. The Southern District Health Board (SDHB) also established an emergency operations centre to ensure that staff were able to travel safely to and from work. The SDHB also assisted several age residential care facilities in Dunedin including finding placement breaks.
During the June flood event, Civil Defence Controller Ruth Stokes confirmed that Civil Defence's Dunedin call centre had received over 3,000 calls over the last 24 hours. On 4 June, Civil Defence visited over 200-flood damaged properties. On 4 June the Fire Service also responded to 345 events, with 90% being in the South Dunedin area. 20 additional Police officers were deployed in South Dunedin to deter looting. In addition, Fire Service Area Commander Lawrence Voight confirmed that firefighters had responded to 130 calls for assistance.
The New Zealand Army was also placed on alert in Mosgiel in the event that the Silver Stream burst its banks. On 3 June, an Army Unimog was used to evacuate children from the flood-stricken Abbotsford School. The Army also helped volunteers sandbag 100 homes in the coastal Dunedin suburb of St Kilda in response to flooding. Houses in South Dunedin's Cutten Street were evacuated with New Zealand Red Cross volunteers visiting households to conduct wellness chats.
Local government On 3 June, the Civil Defence Welfare Committee and Dunedin City Council also established a welfare centre at South Dunedin Presbyterian Church to provide advice and assistance to affected residents. The New Zealand Red Cross, Police and DCC also visited residences and properties in flood-affected areas of Dunedin to provide safety and welfare checks. The DCC and emergency services also discouraged motorists from driving in flood-affected areas to avoid creating "bow waves" that push flood waters into properties.