The upper North Island of New Zealand is set to experience heavy rain, severe gales and big swells this coming weekend as a sub-tropical low approaches, according to the country's weather authority, MetService.

Auckland, Northland and Coromandel are under heavy rain and wind warnings as the system is expected to move in on Saturday, with the worst of the weather anticipated to hit on Sunday.

Auckland, north of the Harbour Bridge, is under a yellow rain warning from 3am on Sunday until 6pm that evening.

Northland's heavy rain watch is in effect from 9pm on Saturday until 3pm on Monday.

Coromandel could be hit by heavy rain from early Sunday morning until Monday afternoon.

Severe gales are also likely to hit the Auckland region from 6am on Sunday until 3pm on Monday.

Big swells are forecast for exposed areas along the North Island's eastern coastline, from Northland down to the Bay of Plenty.

MetService meteorologist Jessie Owen warns that more weather watches could be issued as the system develops.

After the sub-tropical low, a northerly flow will remain, bringing warmer air with temperatures around 20C for parts of the North Island.

However, this will also lead to more wet weather, with Auckland forecast to experience eight days of rain from Sunday according to MetService's extended forecast.