Stormy weather could dump up to 120mm of rain in Northland within 12 hours today and create east coast swells up to 4m.

MetService is warning boaties to stay home as a low passes across Northland today, with heavy rain and strong winds expected between 9am and 9pm.

Niwa is warning the approaching storm is similar to ex-Tropical Cyclone Ita, which struck at Easter three years ago inundating coastal communities across the upper North Island and blowing buildings to pieces on the South Island's West Coast.

MetService meteorologist April Clark said the present low could be particularly dangerous for the eastern coast of Northland through to the Bay of Plenty as waves created by strong onshore winds would combine with king tides.


They could lead to waves pushing further inland than normal, she said, which could waterlog some communities.

"Rain will be heavy at times and we expect between 80 and 120mm to fall on the eastern hills. Between 15 and 25mm of rain an hour during peak intensity is expected and anything above 20mm an hour is considered a downpour."

She said heavy rain may cause streams and rivers to rise rapidly, surface flooding and slips were also possible and driving conditions may be hazardous. Strong wind gusts could damage trees, powerlines and unsecured structures.

Ms Clark said the swell was expected to be about 4m during high tide at 10.06am today at Marsden Pt.

"Thursday and Friday, those waves would be encroaching further over land along the east coast. Boaties shouldn't be out in those conditions."

Ms Clark said severe gales between 89 and 102km/h were expected in exposed places of the east coast, with gusts of up to 120km/h.

Rain is expected to ease later today and showers will linger around Friday and part of Saturday.

The wind will change to westerly tomorrow evening which should take away some of the humidity.

Ms Clark said daytime temperatures would still hover in the mid to high 20s this week with overnight lows of 19C to 20C.

Lifeguards on Northland beaches would decide whether beaches they patrolled at Ruakaka, Waipu, Mangawhai and Whangarei Heads would be closed today or tomorrow.

Stephen Butt, in charge of paid lifeguards in the Northern Region, said they received weather reports daily which alerted them to swell conditions.

"If things turn a bit crazy we will have the guys on the beach evaluate the situation and make a decision on the day."