Heavy rain, which has already forced the closure of Fiordland's Milford Track, will make its way up the South Island tomorrow, accompanied in places by powerful winds.
Much of the South Island is forecast to experience wind and rain through the day, but it should be fine for most of the North Island.
A severe weather warning remains in place for heavy rain on the west of the South Island until late tomorrow, with significant spillover expected into the Otago and Canterbury headwaters.
Environment Canterbury said up to 400mm of rain was expected along the Alpine Divide today and tomorrow, and Environment Southland said the headwaters of the Mataura River and the Oreti River were both rising rapidly this afternoon.
Trampers and campers in the area were warned to stay away from river beds.
The Milford Track was closed today after more than 250mm of rain fell in the region overnight.
Department of Conservation spokesman Ross Kerr said the Milford Track's accommodation for independent walkers was full, and those who stayed in huts last night would spend another night in them tonight.
No new walkers started the trail today.
It was hoped that the weather would clear enough tomorrow so walkers could be helicoptered to safe parts of the track.
Other walking tracks in the area remained open.
MetService duty forecaster Larissa Marintchenko said the heavy rain which has plagued Fiordland and Westland today would spread north to Nelson and the Buller region tomorrow, where about 120mm of rain could fall.
But she said most of the North Island would be "absolutely fine" tomorrow.
However, Wellington and southern Wairarapa would not escape the bad weather, with gusts up to 140km/h expected to blast the south of the North Island overnight.
A severe weather warning for gales remains in place for inland parts of Otago and Canterbury, Wellington and southern Wairarapa today and tomorrow.
The strongest winds - gusting up to 140km/h - were likely around inland parts of central and north Otago, as well as around inland Canterbury.
Canterbury should also expect rain, Ms Marintchenko said.
Environment Southland is warning farmers that as the flood peaks travel downstream it would also affect the middle and lower reaches of the Mataura River and the Oreti rivers, so farmers should consider whether they needed to move stock.