Rain is setting in along the West Coast this morning, with more than a month's worth of rain forecast to lash the region in the next 24 hours.
Almost 40 millimetres of rain has fallen since 1am, and MetService has warned more than 500mm is expected to arrive by Thursday morning.
Orange level warnings were in place in Westland and Buller, with an upgrade to red being considered.
NIWA meteorologist Chris Brandolino said if the forecast volume of rain materialised it could lead to flooding and slips.
It was the result of an atmospheric river that is a channel or corridor of moisture that comes from the tropics and subtropics.
"That's kind of the fuel and then we have low pressure over the Tasman that is harnessing that fuel into rainfall ... several hundred millimetres over a day or so, that intensity could really lead to some impact, in terms of the flooding, slips and things off that nature."
Read this explainer on atmospheric rivers
The weather pattern was also having an impact on the east coast, where foehns (dry warm winds) would drive temperatures up to the high 20s in the South Island and as high as 30 in the North Island.
There was also the chance the strong winds could be damaging in the east, especially around the Canterbury high country tomorrow morning, Brandolino said.
If West Coast rivers flooded, they could spill over into the Canterbury headwaters, down to the Canterbury plains.
More rain was needed, Brandolino said, because he was expecting the South Island to be dry in the main through the rest of spring into summer.
Agencies on alert
Civil Defence manager Claire Brown told Morning Report that even by Coast standards a huge amount of rainfall was forecast to fall in the next day.
However, with roughly 40mm of rain so far, Brown said it was far too early to consider evacuations.
Agencies have been working together for two days to prepare - with vulnerable parts of Westland uppermost in mind, she said.
"It's just about taking precautions, keeping up to date with what's happening ... just keeping a close eye on things and how they evolve over the morning."
She expected all roads to be open at this stage.
She agreed the Coast has had a rough time with bad weather in the last 18 months but the positive was that agencies and communities had gained "a lot of practice" in dealing with adverse events.
'A very wet time'
WeatherWatch forecaster Phillip Duncan said a significant amount of rain was due to fall in a narrow area around Hokitika and Greymouth southwards.
Hokitika was due to receive 120mm of rain in 24 hours and the mountains may receive double that.
"So that's a lot of rain falling in a fairly short amount of time and this is part of a storm system that's been in Australia for the last few days. So a very wet time coming up for the West Coast."
One positive was that the heavy rain was due to fall in an area that could cope with it quite well - the braided rivers that could handle a massive volume of rainfall.
"It's an area that's naturally built for these sorts of downpours."
However, with this event due to deliver 400 to 600 percent of normal rain there could still be slips and other problems.
Two positives - the latest rain was due to last just one day and the long-term outlook for the West Coast was for it to be drier than normal.
In Northland, Auckland and Bay of Plenty humidity levels have been very high, Duncan said, while the overnight low in many parts of the North Island tonight will be 17C, more akin to the middle of summer.
RNZ
