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What you need to know:
- A State of Emergency has been declared in several South Island regions, including Christchurch, Dunedin and Timaru.
- Over 200 residents in Canterbury and Otago have been evacuated from their homes due to the rising flood waters.
- Several state highways are closed including State Highway 1 from Rolleston to Rakaia and SH 75 from Halswell to Little River in Canterbury.
- SH 88 from Maia to Sawyers Bay and SH 8 from Raes Junction to Milton are closed in Otago.
South Island drivers have been warned to take extra care overnight, as sub-zero temperatures teamed with leftover flood waters will make for treacherous conditions.
Metservice tweeted this evening warning drivers to "watch out for frost overnight".
"All the leftover rain and freezing temps [make] a recipe for black ice."
Police are also urging people to stay home during the state of emergency, as some are heading to just to see the damage – and are getting caught in the process.
Police are urging tourists hoping to get glimpse of floods hitting the South Island to stay home, as Christchurch and Selwyn become the latest regions to declare a state of emergency.
- Video: Waist-high floodwaters render Canterbury roads invisible
- Heathcote River floods, leaves residents stranded
Police say some drivers are going out just to see the damage - and are being caught out in the process.
"Police have attended a number of jobs this morning where residents are only travelling to see the damage and witness the wild weather," Senior Sergeant Steve Bothamley said.
"Please be aware that by heading out in these conditions, you could end up taking valuable emergency services away from critical rescue operations."
He urged drivers to aovoid all unnecessary travel.
Several state highways across the South Island remain closed, due to treacherous conditions.
In the Canterbury region, State Highway 1 from Rolleston to Rakaia is closed from the flooding, with State High 77 offered as an alternate route. SH 75 from Halswell to Little River has also been closed until further notice.
In the Otago region, SH 88 from Maia to Sawyers Bay is closed from the slips and flooding, with no detour currently available. SH 8 from Raes Junction to Milton is also closed. Drivers should keep track of updates on the NZTA website.
In Christchurch, the torrential rain and rising floodwaters over the last 24 hours has resulted in the Selwyn River, which was bone-dry just a few months ago, bursting its banks - and now Christchurch has concerns about the damage that could occur.
Christchurch Mayor Lianne Dalziel says declaring a state of emergency is a precautionary measure aimed at allowing the council to prepare for issues brought about by rising tides this afternoon.
The Christchurch City Council says the declaration will give them time to work with agencies and carry out evacuations if required.
Civil Defence and emergency services are working to evacuate residents in Selwyn, as well as another 120 huts near the banks of the river. The evacuation point is at Lincoln Event Centre.
Flooding of the Taieri River has been labelled the second-most serious in its history by the Otago Regional Council, and is thought to remain very high for a long time.
Wastewater systems have overflowed, and those in the affected region have been told not to go outside of their properties unless absolutely necessary.
A boil water notice has also been put in place for Hororata, Acheron, Springfield and Malvern Hills - both Hartley's and Dalethorpe. While no E. Coli has been detected in its water supply, Selwyn Council says it can no logner guarantee the safety of its water.
In Timaru, boil water notices have been issued for the Te Moana, Seadown, Rangitata Huts and Downlands Water Schemes - including Pareora and St Andrews.
Earlier, police offered more details on the nine people who were trapped in flooded cars on Dunedin's State Highway 87, saying they snubbed safety warnings and could have died.
The group of locals, who were awaiting rescue by way of a helicopter, were forced to sit in their vehicles, with some even sitting on top of the cars to avoid the floodwaters.
Police say every member of the group, each in their 20s, was intoxicated and suffering from hypothermia - some in such an advanced state that they needed to be carried out.
They confirmed that the group had ignored warnings to stay off the road and found themselves in trouble as a result.
"Without the assistance of these rescuers, we have no doubt that lives would have been lost," Inspector Kevin Lloyd said.
The Defence Force have this morning gone out to save others trapped on SH87, while fire crews have been deployed in Leeston, Canterbury to rescue at least one car-load of people, who parked up to wait out the storm.
A helicopter has been deployed to rescue a family and another couple in Henley.
Earlier this morning, emergency services in Saint Clair, Dunedin were in a frenzy when "a significant slip" occurred and damaged at least three cars - but there are believed to be no injuries.
The slip occurred just after 8am following a night of heavy precipitation that saw as much as three months of rain fall in just 24 hours in some areas.
Metservice lead forecaster Michael Martin says rain is set to ease in Otago, Dunedin and southern Canterbury now, as it moves north towards Christchurch throughout the morning.
However a state of emergency is still in place in those regions, which bore the brunt of the storm overnight.
"We had rain in Dunedin through the last several hours - between 7-8mm per hour," he said.
He said some places had experienced between 60 and 200mm of rain in the last 24 hours - with the mountains south of Dunedin's Swampy Summit on the more extreme end of the scale.
Oamaru had 161mm of rain in 24 hours, while Dunedin city saw 107mm. The average rainfall for an entire year in the region is 812mm.
Power has been lost at a number of Dunedin and Central Otago homes, with seven roads left with no electricity. More than 200 homes have been evacuated.
In Timaru, a substantial retaining wall has been destroyed thanks to the wild weather, leaving a Douglas St house teetering precariously on the edge of a landslip.
A statement from Dunedin Civil Defence Emergency Management said the organisation's staff would begin assessing damage caused by the wild weather as soon as first light came.
Civil Defence said there has been a large slip on Portobello Rd, close to King George St, as well as various other slips on the Otago Peninsula, which have made accessing Macandrew Bay difficult.
Dozens of roads have also been closed due to flooding - with some missing flood road signs as Civil Defence have run out, meaning caution is being advised on all the area's streets.
Dog owners are also being asked to be cautious around the Silverstream Catchment, as there are fears possum carcasses poisoned with 1080 could make their way into the water and make it toxic for canines.
Meanwhile rain in Auckland has caused problems on State Highway 1, a slip between Pohuehue and Puhoi blocking the solitary southbound lane.