MetService is forecasting further heavy rain for northwestern parts of the South Island and the lower North Island on the back of two very wet days for those areas.

A low pressure system approaching from the northwest is the culprit behind the expected drenching as it crosses central New Zealand tonight through to Saturday afternoon.

Orange heavy rain warnings have been issued for southern Westland, western Tasman, and Mt Taranaki, while heavy rain watches are in place for the rest of the northwest of the South Island and the Greater Wellington region.

There is still uncertainty surrounding the details of where the heaviest rain around the upper South Island and lower North Island will fall, so it is well worth following updates to the severe weather warnings on metservice.com, especially for those in the Greater Wellington region and northern Marlborough.

"Depending on the exact path of the low pressure centre, there could be major impacts for Wellington, and the area is likely to be upgraded to an Orange Heavy Rain Warning," says MetService meteorologist Dan Corrigan.

"Even if warning criteria is not reached, there is still a risk of impacts like surface flooding, slips, and rising rivers in already sodden areas. These hazards, along with poor visibility in heavy rain, can also cause travel delays and disruptions."

The same low pressure system will also cause strong northerly winds across the central North Island hill country from the Kaimanawa Mountains across to South Taranaki from Friday evening through early Saturday morning (a Yellow Strong Wind Watch is in place).

The northerly air flow is also making for muggy overnight conditions, with temperatures remaining in the high teens in the North Island for the next two nights.

Otago and Southland will be the only regions to avoid the rain with the low, and as it moves away during Saturday and overnight into Sunday, the settled weather will spread to the rest of New Zealand.

However, on Sunday afternoon, a change to strong, cold southwesterly winds will arrive in the deep south, bringing showery weather there with a dusting of autumn snow on the mountains above around 800m.

The cooler temperatures and showers will spread northwards across the rest of the country on Sunday night while blustery southwesterly winds affect exposed coastlines.

The showery weather is expected to become confined to the east coasts during Monday, with plenty of sunny skies opening up across the country.

"Looking ahead to ANZAC day, we have reasonable confidence at this stage that high pressure will bring settled weather for most of the South Island, but it could well be a frosty one for people waking up early.

"Southwesterly winds may still be quite breezy in the North Island, but the weather looks largely dry aside from showers for eastern parts."

Image: MetService.