Nelson, in New Zealand's South Island, has been the worst affected with hundreds of homes evacuated earlier in the week and some residences declared uninhabitable due to rain.

Heavy downpours have also hit the North Island, causing towns to be cut off, submerging roads and houses, and prompting evacuations.

There were about 100 new evacuations overnight across the country, The New Zealand Herald reported on Saturday.

"Things going forward look quite different from this past week," the Metservice weather forecaster said in a Twitter post.

Earlier, it had said warnings and watch alerts were lifted after rain eased in northern and central areas.

Nelson saw more evacuations on Saturday, said Kieran McAnulty, the emergency management minister, who thanked those assisting in the rescue effort but added the area's recovery would be a "long and difficult" process.

"States of emergency remain ... in Nelson-Tasman, Marlborough and West Coast," he said on Twitter.

In the Tasman district neighbouring Nelson, emergency authorities said "many more" evacuations were carried out overnight, with 508 homes now evacuated in the region.

"That looks set to increase even more," Nelson Tasman Civil Defence said on Facebook on Saturday.

The inundated area broadened due to overnight rain and now includes the Cannan-Upper Takaka area and the Richmond ranges.

A warning for heavy rain remains for southern Fiordland in the southwest of the South Island, Metservice said on its website.

Britain's Prince William and his wife Kate said on Saturday they were keeping affected New Zealanders in their thoughts.

"Seeing the devastation caused by the floods in New Zealand is a reminder of the herculean effort we must all make to protect our planet," the royal couple posted on Twitter.

Australian Associated Press