Holiday-makers are being warned of flash flooding and slips as heavy rain and gales hit the north of the country, swamping campsites.

Torrential rain flooded a Department of Conservation campsite at Uretiti Beach south of Whangarei, in the early hours of New Year's Day.

Severe weather didn't stop these people enjoying a swim at Takapuna Beach on New Year's Day. Photo: Jason Dorday/Fairfax NZ.

"It's still raining and we're having up to 40 knot (75kmh) winds and gusting so it's quite miserable in tents," says a staff member at the campsite.

"The rain hasn't given up, it's been quite relentless."

Although there had been no flooding, gazebos had collapsed, she says.

"We have 300 sites and we were full but we've had sort of a mass evacuation. A lot of young people were dropped off by their parents and they just can't cope, the parents have been flooding in to pick them up."

The worst affected areas are expected to be Northland, Auckland and Coromandel Peninsula, with Bay of Plenty and Waikato also taking a hit.

Latest update to Severe Weather Warning and Watch! Read up before tramping/camping (https://t.co/Sd5C6lrsSL) ^AH pic.twitter.com/9nycIVYuG5

— MetService (@MetService) December 31, 2015

Auckland Civil Defence and Emergency Management Director John Dragicevich says surface flooding was likely across Auckland and there was a potential for damage.

"Campers and holiday-makers should check and secure their camping and outdoor equipment, and be ready to move quickly if required."

MetService is warning up to 200mm of rain could fall about the eastern hills of Northland through to late Saturday. Up to 180mm could fall in Auckland mainly north of the bridge and in the Hunua Ranges by midnight Saturday, with 180mm also possible on the Coromandel Peninsula from noon Friday to 6am Sunday.

Rainfall intensity could get up to 35mm an hour in localised downpours, MetService said.

"People, especially those planning outdoor pursuits, are advised this rain will cause rivers and streams to rise rapidly, with slips and flash flooding also possible."

The rain was being caused by a low lying to the northwest of the country.

The leading edge of the rain band reached Kerikeri before midnight and MetService radar showed rain across nearly all of Northland and Auckland by 9am.

MetService warned easterly gales could become severe in exposed areas, with gusts up to 100kmh in Northland and 90kmh in Auckland and Coromandel.

-Stuff.co.nz