After a stretch of sweltering temperatures, a wet and thundery change is set to wreak havoc across the country this week.

MetService said a low-pressure system in the Tasman Sea will move on to central New Zealand tomorrow and Thursday, bringing heavy rain to the upper South Island.

Showers are also forecast to drench the North Island from tomorrow, becoming more widespread as the day progresses.

The heavier rain for the north is expected to arrive on Thursday, alongside a risk of thunderstorms.

MetService meteorologist Michael Pawley told the Herald a second frontal feature will swing across the North Island, bringing a “big region of instability” that could cause heavy falls and thunderstorms.

“The first feature reaches the northern parts of the South Island and then there’s a second part of it.

“It’s sort of got arms. It’s got one arm that reaches forward and then another one that swings through,” Pawley said.

He said the forecast indicated the thunderstorms are most likely to impact western parts of the North Island and they will spread further east from the afternoon.

“There are some risks around the South Island, the Buller, Nelson and Marlborough areas, and then further south along the spine of the Southern Alps, south of Mount Cook to Fiordland,” Pawley said.

MetService meteorologist Mmathapelo Makgabutlane earlier said Tasman, Nelson, Marlborough, Canterbury and eastern Otago are forecast to receive the largest share of the rain this week.

The associated cloud and rain is also forecast to bring a noticeable cool-down from Thursday into the weekend, MetService said.

Pawley said this will create a different feel compared with the past weekend.

“Into the latter part of the week, places like Ashburton and Timaru will only get to 15C as a maximum, whereas they were pretty hot last weekend.

“Hastings is forecast to get to 19C this Sunday,” he said.

This comes after temperatures reached up to 36C over the weekend.

Hawke’s Bay led the national high with temperatures hitting 36.4C in Napier and 36.2C in nearby Hastings on Sunday. Gisborne also got to 33.2C.

Christchurch climbed to 31.2C, while Whangārei and Tauranga fell shy of 30C with highs hovering around 29C.

Watches and warnings

MetService said Nelson and the Tasman districts north of Nelson Lakes will be under a heavy rain watch from 6pm tomorrow until noon on Thursday.

Marlborough and the Kaikōura Coast are also under a heavy rain watch from 8pm tomorrow until 2pm Thursday.

All of these areas are likely to have periods of heavy rain that may approach warning criteria.

MetService said thunderstorms are possible west of Takaka.