Heavy rain and severe northwest gales are set to impact much of the South Island and parts of the lower North Island from tomorrow.

A raft of weather warnings and watches have been issued by MetService.

"An intense front embedded in a strong and moist northwesterly flow moves north over the South Island on Sunday and onto the North Island on Monday bringing heavy rain, mainly to the west of the South Island," the forecaster said.

"Severe northwest gales are likely to affect many South Island areas and the lower North island."

An orange heavy rain warning is in place for the Westland District, Fiordland north of George Sound, the headwaters of the Canterbury lakes and rivers, along with the headwaters of the Otago lakes and rivers, from tomorrow morning into the evening at the latest.

Both the Grey and Buller districts are under the same warning from tomorrow morning until early on Monday, while the warning is in place for the ranges of Tasman west of Takaka from tomorrow afternoon into Monday morning.

Meanwhile, the Tararua Range is under the orange heavy warning from tomorrow evening into Monday morning, where 70 to 100mm of rain can be expected.

MetService said the Westland District can expect 200 to 250mm of rain about the ranges, and 110 to 150mm nearer the coast.

The forecaster also said 110 to 150mm of rain can be expected for the ranges of Tasman west of Takaka, while the Grey and Buller districts can expect 110 to 160mm of rain about the ranges and 80 to 100mm nearer the coast.

The headwaters of the Canterbury lakes and rivers can expect 200 to 250mm of rain about the main divide and 80 to 110mm within 20km farther east, while the headwaters of the Otago lakes and rivers can expect 120 to 150mm of rain about the main divide, and 80 to 100mm within 20km farther east.

Meanwhile, an orange strong wind warning is in place for Canterbury High Country from tomorrow morning into the evening.

The same warning is in place for Marlborough Sounds, Kaikōura ranges and Kaikōura coast, and Wellington and Wairarapa south of Masterton from tomorrow afternoon into Monday morning.

MetService said Marlborough Sounds, Kaikōura ranges and Kaikōura coast could experience a severe northwest gale gusting 130km/h in exposed places, while Wellington and Wairarapa south of Masterton could see that gale gusting to 130km/h in exposed places.

Canterbury High Country, meanwhile, could see a severe northwest gale with damaging gusts of 150km/h in exposed places.

A number of heavy rain and strong wind watches have also been issued.

Mount Taranaki and the ranges of Marlborough west of Havelock are under a heavy rain watch from tomorrow evening into Monday.

"Periods of heavy rain, and amounts may approach warning criteria," MetService said.

Meanwhile, a strong wind watch is in place for Otago, Southland, Stewart Island and Fiordland, along with the Canterbury Plains and Banks Peninsula from tomorrow morning into tomorrow afternoon and evening respectively.

The watch is also in place for Marlborough apart from the Sounds, Kaikōura ranges and Kaikōura coast from tomorrow afternoon into early Monday morning.

Wairarapa including the Tararua District from about Masterton northwards and Hawke's Bay south of Hastings is under the strong wind watch from 2am to 9am on Monday.

MetService said northwest winds may approach severe gale in exposed places in these areas.

The NZ Transport Agency (NZTA) said it is warning those planning on travelling through South Westland throughout tomorrow and overnight that State Highway 6 could be closed at short notice if heavy rain sets off fresh slips or rockfall.

The highway was closed for 12 days after the November 9 deluge as rockfall, slips and flooding were cleaned up.

"We are monitoring the area around Epitaph Slip this weekend and if rainfall does intensify on Sunday, we may close that section of route on Sunday, 8 December, either because of fresh activity, or as a precautionary approach. This will be the largest rainfall since we had the original issues there," the agency's Central South Island system manager Mark Pinner said.

He also warned strong winds were forecast for the Canterbury Plains, including State Highway 73 where firefighters were still battling a large vegetation fire, Banks Peninsula, the Kaikōura coast and top of the South Island/Wellington (SH1).

"People need to be on the lookout for downed trees or branches over the road in these conditions. NZTA crews will be doing their best to keep these routes open and clear," Pinner's statement said.

"People driving high-sided vehicles like camper vans, towing or riding motorcycles are advised to take care and park up if in doubt in these circumstances."

People can keep up to date with the latest forecasts on MetService's website.