Metservice data showed Nelson city on the South Island had received 106 millimetres of rain since midday on Tuesday - well above its average rainfall for the whole of August of 80 millimetres.
On New Zealand's North Island, the country's largest city, Auckland, is under a heavy rain and winds alert, with minimal disruption reported so far.
Authorities said more than 230 homes in Nelson, a city with a population of more than 50,000, have already been evacuated with many public facilities and roads are closed.
A statement on Nelson City Council's website warned continuing rain could mean more land slips, flooding and evacuations.
Nelson's Mayor Rachel Reese told New Zealand television show AM that while the city had made it through the night without any major incidents, infrastructure was under pressure.
"We are dealing with a lot of wastewater overflows," she said.
On the west coast of the island Buller District Council said in a statement people from 160 homes evacuated over the last day were able to return to their residences to assess damage. But it warned further rain was expected and it was possible that they would have to evacuate again.
"Right across the district I believe we got away relatively unscathed," Buller Mayor Jamie Cleine told a news conference streamed online.