People in northern New Zealand are being urged to prepare for heavy rain which is set to hit the country for Auckland Anniversary weekend.

MetService has issued heavy rain and strong wind warnings for parts of the North Island.

Significant rainfall is expected in Northland, Coromandel Peninsula and western Bay of Plenty this weekend.

A heavy rain watch and a strong wind watch were also in place for Auckland.

Waka Kotahi has placed a strong wind watch on the Harbour Bridge, with winds gusting 40-50km/hr, strengthening this evening to 50-60 km/hr.

Drivers of high-sided vehicles and motorcyclists are advised to avoid the bridge.

An amber alert is in place with 90kmh winds forecast tomorrow from 7am to midday. Under this alert, speeds are reduced and lanes may close.

And forecast winds of 90-100kmh with a risk of 110kmh would signal a red alert from midday to 10pm, closing all lanes on the bridge.

Planned overnight maintenance has also been shifted to Sunday due to weather.

Aucklanders planning to travel on Friday should consider delaying as the weather would mean an increased risk of delays, especially around Coromandel, MetService said.

"More cars on the road, tired drivers and people driving on unfamiliar roads can make holiday driving stressful and risky," Waka Kotahi director of regional relationships Steve Mutton said.

"Checking your car is safe before your journey, keeping your speed down, driving sober, watching for the signs of fatigue and sharing the driving are just some of the things you can do to ensure you arrive safely at your destination."

MetService said the humid northerly flow set up over the country was here for the long run and warm air from the north will raise termperatures.

"It is going to be a very wet start to Auckland Anniversary weekend," MetService meteorologist Jessie Owen said.

"Heavy rain could cause flooding and slips and make travel hazardous so if you're planning on heading away for the long weekend it's a good idea to keep up with the forecasts and perhaps consider delaying your travel."

The humid air from the north could bring further bouts of heavy rain to northern areas, and to the west of the South Isalnd, into next week, Owen said.

Coastal water temperatures were warmer than what were expected for this time of the year, MetService said.

Severe marine heatwaves were present in Cook Strait, Chatham Islands, Banks Peninsula and Stewart Island.

Several marine warnings had been issued, with gale warnings in place for Bay of Islands, Bream Head to Cape Coville and Puysegur, and strong wind advisories for Hauraki Gulf, Waitematā Harbour, Manukau Harbour, Coromandel, and Bay of Plenty.