Weather: New Zealand set for 'cold blast of air' this weekend ahead of warm, sunny conditions next week

Kiwis are warned it'll be a chilly next few days as south-westerly winds make their way up the country, bringing cool temperatures.

After a series of muggy nights over the last week, forecasters say overnight temperatures are set for a sharp drop.

The windy west-to-south-west change will be most noticeable in the lower South Island on Friday night, Weather Watch says - and could cause inland areas to plummet below 0C.

From then, a "cooler than average change" will spread behind it across Friday and Saturday.

"Friday will see the South Island lean cooler than average in some regions, like Southland, some parts of Otago and Canterbury and the more populated parts of the West Coast," Weather Watch says.

"In the South Island, inland and through the high country of Canterbury, there is even the chance of a frost… Any frosts will be very limited to inland areas through the mountains."

Head forecaster Philip Duncan said some parts of the South Island won't even get into the teens on Saturday.

"This is a cold blast of air for this time of the year."

In the North Island, conditions will stay humid through Friday but will drop noticeably overnight heading into Saturday.

"All of New Zealand will either be below average, or about average, over Friday night. No one will be warmer than average for the first time this week," the forecaster says.

While it may be cold overnight, it'll be clear skies and sunny conditions for most eastern and northern parts of the North Island over the weekend.

A ridge of high pressure in the Tasman Sea is building and will track over New Zealand next week, bringing settled, warm conditions.

Earlier this week, it was revealed a cyclone is brewing in the Pacific Islands and could lash New Zealand in the lead-up to Christmas.

Modelling shows the weather system will gain in strength between Fiji and Vanuatu mid-next week before pivoting into the Tasman Sea and towards Aotearoa around the summer solstice on December 21.

"New Zealand is in the risk zone for it," Duncan says. "It's not guaranteed to hit us - not at all - but this far out, to be seeing a storm like that in this area, it means it's one for us to keep a close eye on."