Residents of low-lying areas of Gore and Mataura are being evacuated as river levels rise and people in a third Southland town, Wyndham, are being told they will have to leave.
Teams of police went door to door this morning to ensure people were leaving. Large swathes of farmland were underwater and state highways in the area are closed.
Power is off to east Gore, and the main bridge over the Mataura River in Gore is closed to all but emergency services, Civil Defence said.
Police were also helping evacuate people in Mataura.
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At 5.30am the Mataura River was 4.3m above normal levels at Gore and 6.5m above normal just south of Mataura. Large swathes of farmland were under water and many roads are impassable.
Southland civil defence officials told people in low-lying areas of the towns to leave their homes and go to friends and family on higher ground or to community hubs set up for evacuees in both towns. The community centres in Gore filled up this morning and residents were being sent to Gore High School.
Wyndham was likely to flood this afternoon and residents should go towards higher ground at Mokoreta, officials said.
Riversdale residents were put on standby to leave their homes if required.
Anyone who needs help should call 111.
Southland Civil Defence Emergency Management Group chair Neville Cook said it could be several days before rivers in the area returned to normal, so people should take medication, a change of clothes and personal items. Pets can be taken to the Waimumu Field Days site.
"Please don't try and drive in the flooded areas because we suspect in some areas roads will be washed out and the water will be contaminated.
"Fortunately we've had time to plan this, everything is in hand now. The police and the armed services and fire and various other groups are assisting with the evacuation."
Floodwaters were due to peak about 8.30am in the Gore area and at 10.30 at Mataura, Cook said. "These peaks may last for a while - it's not going to be a five-minute thing."
"Please take essential documents, personal medications and clothing with you. If you require assistance, please call 111."
An update posted just after 9am said the evacuation of almost 200 tourists at Milford Sound got under way at first light this morning.
Helicopters are being brought in to help with the evacuation. Buses will take tourists to the Fiordland Events Centre where they will be met by Emergency Management Southland welfare staff.
"Morale has been high among the visitors and staff, as they received regular briefings and have been in contact with friends and family."
Electricity has been cut to east Gore due to heavy flooding in the area and a "boil water" notice has also been issued by the Gore District Council.
There is a potable water tanker at Gore High School for those who require it.
"The Riverton Highway is likely to be significantly affected, so please bear this in mind when planning any travel.
"We advise avoiding any unnecessary travel - taking extreme caution if you need to go out and checking [the NZ Transport Agency] and local council websites for road closures."
Up to 15 homes were earlier evacuated on Ontario St in Gore.
UPDATE 6AM 5/2/20 - EVACUATE NOW. Gore and Mataura residents in the areas identified in the maps below should evacuate...
Among those residents is Hokonui Breakfast radio host Luke Howden and his family.
He described having to wade thorough waist-high water to leave their home late yesterday afternoon.
"It would've been about 4 o'clock yesterday that the water just started to rise quite rapidly and before we knew it, it was licking on the back doorstep.
"Every major trunk line in and out of Gore has been closed. As a result, you don't have the through-traffic that you usually would at this time of the morning.
Flooding is continuing to cause issues throughout Southland. Road closures will continue throughout the day, with more...
"Yet in the background, you can hear the surging torrent of the Mataura River. I went for a wee walk down the banks moments ago and, at this stage, it looks contained.
"I'd say there's probably about a 2m clearance between the water level and the bottom of the Gore Bridge - which, at this stage, remains open to link traffic between east and west Gore."
Trapped on roof by high water
A carload of people had to be rescued from the top of their vehicle in Otago after getting into trouble on a flooded southern road yesterday.
A Fire and Emergency New Zealand spokeswoman said they received a call at 12.50pm that some people had driven down a road near Kelso.
The road was at that time so flooded rising waters looked like they could inundate the car.