The South Island's West Coast is bracing for another big soaking, with hundreds of millimetres of rainfall expected later this week.

MetService has issued an "orange" heavy rain warning for Westland throughout Wednesday and into early Thursday morning, when 200mm to 300mm is forecast about the ranges south of Otira, with peak rates of 20mm to 30mm per hour.

The ranges between Ross and Bruce Bay could see 300mm to 400mm, or possibly more – while about 100mm to 150mm was expected in the wider region, with the potential for swollen rivers, slips and surface flooding.

MetService meteorologist John Law said that was a "huge amount" of rainfall to move through – even for a typically wet region like the West Coast.

It was being carried by a low pressure system dragging warm, wet air towards New Zealand from the northwest.

The agency was keeping the warning level at orange for now, but was ensuring local authorities stayed updated.

"We're going to be keeping a very close eye on this and we work very closely with regional councils and their teams along the western coast as well."

The mid-week deluge would be accompanied by strong northwesterly winds that would push some of the rain over the Southern Alps and into the headwaters of Canterbury and Otago lakes and rivers.

"So, while we're looking at the heaviest rainfall on the western side of the country, even in the east, we'll find some of that rain making its way to those regions as we go through Wednesday and Thursday," he said.

"We can also expect some pretty windy conditions on the eastern side of the South Island, with perhaps severe gales in the most exposed areas of the Canterbury high country, for example."

In Buller, the ranges of Tasman and western Marlborough, MetService had "moderate confidence" that a rainfall warning would be needed on Wednesday, but high confidence for Thursday.

For eastern Bay of Plenty, meanwhile, there was low confidence that the front would bring warning amounts of rain on Thursday.

Elsewhere in the North Island, severe gale northwesterlies were possible for Marlborough, Wellington and Wairarapa on Wednesday and Thursday.

Following the front, a cooler southwest change was expected on Friday.

While that could bring some snow to the higher roads of the southern South Island, it was unlikely that snow warnings would have to be issued.

"However, there is low confidence that strong southwesterlies behind the change will require a warning for Stewart Island and parts of coastal Southland and Clutha on Friday," MetService reported.

Source / MetService

"A large high should begin moving onto the country during Saturday, bringing easing conditions."

As for today, WeatherWatch was picking the potential for thunderstorms in some places today.

"Some cold upper air lies over New Zealand today especially the South Island, relatively good surface moisture exists and winds are converging," the company reported.

"This leads to a chance of unstable conditions and thunderstorms this afternoon for both Islands.

"Upper-level wind shear is strong over North Canterbury so this could throw up a small twisty surprise perhaps."

A chance of hail existed with any storms for the South Island also, mainly about Canterbury, starting out around midday for South and mid-Canterbury then progressing further north into North Canterbury during this afternoon.