A rare Red weather warning has been declared for western parts of the South Island which is forecast to get heavy dumps of rain from Wednesday.
MetService says the amount of rain in the Westland and Buller ranges until at least Friday was expected to cause dangerous river conditions and significant flooding. Slips and floodwaters were likely to disrupt travel, making some roads impassable and possibly isolating communities.
Meteorologist Angus Hinds said this afternoon that the rare Red severe weather warning has been declared for Westland and Buller in consultation with regional councils.
"Red warnings are reserved for our most impactful weather events - they do not occur often around the country - so when it's a Red warning, you know things are about to get serious."
Hinds said flash flooding was possible because rain was falling on the back of record dry weather in January, causing waterways to swell up rapidly.
There was also a widespread risk of slips and and landfalls which could shut roads. Hinds said high tides at 1am and 1pm were already "very, very high" and heavy rain would put added stress on the drainage system.
In Westland from 1am on Wednesday until 7pm on Thursday, between 550mm and 750mm of rain - up to 75cm - was forecast to accumulate about the ranges, and 150 to 250mm near the coast, but possibly 300mm near the Glaciers.
"Even by the wet standards of Westland, that is an incredible rainfall event," Hinds said.
There would be peak rates of 30 to 45mm/h about the ranges, with thunderstorms. Further rain was likely during Thursday night and Friday, and the Red warning could be extended.
In Buller in the 54 hours from 6pm on Wednesday, expect 300mm to 400mm of rain to accumulate about the ranges, and 150mm to 200mm near the coast.
There would be peak rates of 20 to 35mm/h about the ranges, with thunderstorms possible. More heavy rain was likely during Friday night and Saturday, and the Red warning could be extended.
West Coast Civil Defence Emergency Management regional director Claire Brown told RNZ's Morning Report programme today the region began planning for the rain over the weekend alongside MetService.
"We're talking to the community coordinators around the Westport area, but we're all very concerned about what could eventuate."
Brown said the region's activation centre was expected to be up and running today.
Warning for Southern headwaters
Heavy rain warnings for parts of the South Island have also been issued, and streams and rivers also on the east of the island may rise rapidly due to the exceptional amount of rain up in the mountains, MetService's Angus Hinds warned. Surface flooding and slips were also possible, and driving conditions may be hazardous.
For the headwaters of Otago lakes and rivers: From 3am on Wednesday until 3am on Thursday, expect 200mm to 300mm of rain about the divide, and 100mm to 150mm within 15km east of the divide. Peak rates of 20 to 35mm/h about the divide.
For Fiordland, mainly north of Doubtful Sound: From 6am on Wednesday until 3am on Thursday, expect 100mm to 130mm of rain. Peak rates of 15 to 25mm/h.
For the headwaters of the Canterbury lakes and rivers: From 7am on Wednesday until 7pm on Thursday, expect 350mm to 450mm of rain about the divide, with 150mm to 200mm within 15km east of the divide. Peak rates of 20 to 40mm/h expected about the divide, with thunderstorms likely.
A strong wind watch was also in place for the Canterbury High Country from 9am to 10pm on Wednesday. Northwest winds may approach severe gale at times.
"So, this will be a high impact rain event for a traditionally wet area of the motu, that is the West Coast of the South Island."
But even areas between Abel Tasman down to Milford Sound could expect up to 700mm of rainfall over multiple days.
"That will certainly increase the risk for slips, flooding. So people who are tramping ... certainly want to reconsider your plans if not put them off."
Brandolino said tropical fronts from Australia would be bringing the rain down from the Timor Sea.
"It's an atmospheric river, in fact, it may be a couple atmospheric rivers.
"An atmospheric river is kind of a stream ... the name might suggest of moisture coming from the tropics and subtropics so it's the transport of unusually high moisture content. And oftentimes this is responsible for New Zealand's big rainfall events and we get this transport of tropical moisture."
Brandolino said the wet weather may also affect Wellington and the eastern South Island at the start of the long weekend.
"But we're hopeful things improve for the second half of the long weekend."
- ODT Online and RNZ
