Heavy rain and strong northerly gales are settling in over the upper South Island as a slow-moving front moves across the country, with the North Island to bear the brunt from tomorrow.

MetService today kept in place a severe weather warning for the upper South Island and lower North Island, after the front passed over Fiordland and Westland overnight.

Severe weather forecaster Heath Gullery said about 90mm of rain had fallen at Milford Sound, which was "about right'' compared with forecasts.

The front was continuing to move up the country as expected, with heavy rain and strong gales coming ahead of it.

"The front that's bringing the heavy rain is currently over the upper South Island so we're getting heavy rain into Buller and parts of Nelson and Marlborough," Mr Gullery told NZPA.

The Nelson Tasman Civil Defence have issued a flooding alert for the Aorere River in Golden Bay, after 160 millimetres of rainfall in the past 24 hours has pushed the level of the Aorere River to 4.9 metres as of 10.30am today.

Further rain is forecast for this area, and the Civil Defence is advising people close to rivers and streams in the Aorere Valley to prepare for a rapid rise in water levels. Farmers should move their stock to higher ground where necessary.

The ranges of western Nelson and northern Marlborough were expected to receive about 180mm of rain, with up to 150mm expected in the Buller ranges.

"That heavy rain is expected to continue right throughout today and then clear tomorrow morning," Mr Gullery said.

"It's basically stalled across the upper South Island throughout today, and then the front makes slow progress across the North Island tomorrow."

Heavy rainfall warnings remained in place for the Tararua Ranges and Mt Taranaki from tonight, while Tongariro National Park and the eastern Bay of Plenty ranges would receive warning levels of rain from tomorrow through Friday.

Rivers and streams were likely to rise quickly and surface flooding and slips were possible in badly-hit areas.

Mr Gullery said a comprehensive weather watch covered most other North Island areas for either heavy rain or strong winds.

WeatherWatch's head analyst Philip Duncan said the front was "one of the more explosive and clearly defined fronts that we have seen this year".

"It is not a sub-tropical low, but it is pulling into the subtropics, so humidity is going to jump up tonight an that northerly flow that is coming out of the sub-tropics is going to really invigorate those clouds bringing some very heavy torrential rain at times in the North Island.

Mr Duncan said the Bay of Plenty, particularly inland and south-eastern areas, will be particularly hard-hit, as well as the King Country through to the Central Plateau, the Tararua Ranges, Mt Taranaki region and the Nelson Ranges.

"There is also a moderate risk of isolated thunderstorms over parts of the North Island tomorrow," he said.

He said the rain is likely to start to clear most places by tomorrow night and Friday morning.

"The weekend is not looking too bad, there could be a few coastal showers on the West Coast and Southland, but overall most of the country is not looking too bad."

- additional reporting NZPA