An "atmospheric river", as described by forecasters MetService, from the north has brought widespread rains to the country this week.
The worst-hit city so far is Nelson, where rainfall has destroyed roads and a state of emergency has been declared.
Authorities ordered 233 homes in Nelson East to evacuate overnight as the Maitai River swelled.
Returning to inspect her property on Thursday afternoon, resident Maggie Grey told Radio NZ there was an inch-thick sludge in her home.
"I don't know what to expect given the weather isn't over yet," she said, with other homes on her street appearing worse-off.
On Tuesday, MetService issued a rare red warning, saved for the most damaging weather events, for Buller and Westland on the west coast of South Island.
They added the Nelson region to the list late on Wednesday, warning of worse rain to come.
Nelson is one of the sunniest regions of New Zealand, receiving an average of 960mm of rainfall each year.
It was soaked by 89mm on Wednesday alone, with predictions of 200mm more this week.
In the often-soaked Westport, which received 70mm on Wednesday, about 160 households were encouraged to "self-evacuate as a precaution" by civil defence.
Another front is due to arrive on Friday.
"This is a chance to take a breath and set ourselves up for what comes next ... don't get rid of those sandbags yet!" Buller Emergency Management posted on Facebook.
The system has also brought heavy rain and savage winds to Northland, north of Auckland, where flooding has closed State Highway 1.
The regional capital of Whangarei, as well as Auckland, had their warmest August nights on record on Wednesday night.
Under La Nina conditions, climate agency NIWA said last month was New Zealand's wettest July on record.
Christchurch suffered its wettest month on record, with three major rains bringing floods and about six months' average rainfall to the Garden City.
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