Most of the country is now on alert amid an ongoing "significant rain event".
The weather system was forecast to move north on Thursday, with many parts of the North Island now in the firing line.
MetService has issued several weather warnings and watches right across the country.
The warnings and watches cover most of the South Island, except parts of the east coast, the lower North Island, Auckland, Northland and Bay of Plenty. These include a severe thunderstorm watch for the upper North Island. Snowfall warnings have also been issued for two South Island roads.
"This is a significant event for Westland, with potential for upgrade to a Red Warning," MetService said.
"People should listen to advice from local authorities, and those who need to travel through the region need to take extra care."
The forecasting agency added a significant amount of rain is expected in Southland and localised flooding is likely, especially in the west.
The article continues after Newshub's live updates.
West Coast Emergency Management group controller Te Aroha Cook told AM there was potential for a further 300mm of rain to fall on Thursday until at least 8pm.
"Our priority of concern is still the south Westland, that's from Franz Josef through to Haast, with the greater intensities falling at Franz Josef," Cook said.
Residents have been urged to be prepared for possible evacuations and a boil water notice has been issued for Franz Josef residents.
Cook added there are two rivers at alert levels - the Waiho and Hokitika.
She said there is a likelihood of surface flooding in low-lying areas.
"Everything in the district is holding up pretty well as we keep saying to people the West Coast and Westland is open for business and all we ask is people do monitor weather conditions and check road conditions."
