Cyclone Gita is coming - and the South Island is expected to come off worst on Tuesday.
NIWA forecaster Chris Brandolino told The AM Show Monday "it will be a rough day."
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"Wellington especially south to Buller, Nelson, Motueka, Westport, even towards Christchurch - they've got a good drop of rain," he says.
"Blenheim as well. So far southern North Island and the northern half of the South Island."
Mr Brandolino warns the "short and sharp" weather could on Tuesday hit right at rush hour.
"I think tomorrow the impact in the Wellington area could be quite significant," he told RadioLIVE.
"There could be some very heavy rain that falls at the wrong time... rush hour - so there could be some high impacts there."
WeatherWatch's head forecaster Philip Duncan agrees, warning the storm will be damaging.
"The area that I'm most concerned about is the West Coast. I think they could be in for some very strong winds, damaging gusts," he says.
"And then we've got this really intense heavy rain crossing over the South Island as well."
He says rain will be worst around the area north of Westport and Nelson - while Dunedin and Canterbury are a flood risk.
"Canterbury could be dealing with some very intense rain, and the numbers are anywhere between 100mm and 200mm," Mr Duncan says.
"I could see winds gusting well over 120k in exposed areas, and that will cause some damage."
Waves are expected to be over six-metres high, along with the gale-force gusts.
Fortunately, the upper North Island will escape.
"There's not going to be much north of that," Mr Duncan says.
"Here in Auckland, north of the Bombays, not much of an impact."