It is about to bucket down in parts of the country over the next couple of days with more than half a metre of rain in places potentially triggering flooding and slips.

The MetService is forecasting the largest accumulations for parts of the South Island with Fiordland and West Coast set to take on 600mm between Sunday and Tuesday - more than half the annual total for Auckland.

Forecasters are warning the big wet could trigger slips, flooding, road closures and detours.

"A change is coming for many this week, as a cold front moves over the South Island and lower North Island," MetService meteorologist Rob Kerr said.

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"Warm, moist northwesterlies ahead of the front are set to bring heavy rain and gale force winds to southern and central parts of the country during the next few days."

On Monday morning MetService meteorologist Stephen Glassey said rain had started falling in Fiordland overnight with 54mm recorded at Milford Sound between 12am and 5am.

Heavier falls were expected later today but the biggest deluge was due tomorrow, Glassey said.

Otago and Canterbury headwaters could also expect significant rainfall with 500mm possible about the divide and 280mm within 20km of the divide during tomorrow and Tuesday.

The cold front would slowly sweep over the South Island before stalling over the North Island about midweek.

Glassey said once it arrived in the North Island from Wednesday it would have severely weakened but still bring some rain and showers to places.

In Auckland, weather consisting of cloudy periods and isolated showers could be expected over Monday and Tuesday with temperatures hovering around 24C.

"As [the front] stalls, it's going to bring more rain and shower activity across the North Island," MetService meteorologist Josh Griffin said.

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"In Auckland ... there is some rain developing late Wednesday into Thursday and it'll kind of be cloudy and a bit showery for the Auckland area towards the end of the week."

However, the time has not yet come to pack away the shorts and T-shirts with New Zealand's long, hot summer set to linger.

Temperatures would remain consistently in the mid to high 20s for the upper North Island and in the low 20s for much of the rest of the country.

On Monday eastern parts of the country were in for some warm and dry weather. In the South Island the east coast was set to cook, thanks to the foehn effect driving warm and dry air from the Southern Alps. Christchurch was forecast to hit 29C.

A high-pressure system would bring settled weather to the country this month and despite the arrival of the cold front midweek, the weather will remain warm.

The continuing mostly settled, warm weather might be bringing smiles to many, but it was no anomaly.

"It seems to be nothing out of the ordinary for this time of year ... it's not going to be getting cold just yet," Griffin said.

Today's weather

Whangārei

Cloudy periods, chance shower. Northeasterlies. 24C high, 16C overnight.

Auckland

Cloudy periods. Isolated showers, mainly morning and night. Northeasterlies. 24C high, 17C overnight.

Hamilton

Some morning cloud then fine. Chance afternoon shower. Light winds. 25C high, 12C overnight.

Tauranga

Mostly cloudy, a few morning showers. Northerlies. 23C high, 17C overnight.

New Plymouth Cloudy at times, possible shower. Northerlies. 23C high, 17C overnight.

Napier Fine. Light winds and afternoon northeasterlies. 27C high, 16C overnight.

Whanganui Fine, apart from some morning cloud. Northwest breezes. 26C high, 16C overnight.

Wellington Mostly cloudy, isolated showers possible in the north. Strong northerlies. 21C high, 17C overnight.

Nelson Fine, possible showers from afternoon. Northerlies developing. 23C high, 17C overnight.

Christchurch Early fog otherwise fine with high cloud. Northerlies. 29C high, 19C overnight.

Dunedin Occasional rain developing this morning. Northerlies. 24C high, 19C overnight.