About 150 campers in the Bay of Plenty have been forced to evacuate amid rising floodwaters in the early hours of this morning.

More than 100mm of rain was dumped on parts of the Coromandel overnight - and more rain is to come, with the wet-weather system bearing down on the north labelled a tropical "firehose" of moisture by the National Institute of Water and Atmospheric Research.

Police, firefighters and Bay of Plenty Civil Defence were called to Waihi Beach from 2.50am when campers phoned emergency services due to flooding.

A police media spokeswoman confirmed officers were helping to evacuate campers from the Waihi Beach Top 10 Holiday Resort for about 40 minutes. Firefighters were understood to still be there helping campers at 4am, she said.

The campground is a popular Christmas holiday spot.

The spokeswoman said the conditions involved "quite bad flooding" and Civil Defence was notified.

MetService meteorologist Andy Downs told Tim Dower down the road in Tauranga, there's been very heavy rain this morning.

"Tauranga Airport's already received 90 millilitres of rain, with over 23 millilitres in oen hour earlier this morning. That is a lot of rain."

St John ambulance staff were called, but stood down soon after.

She said there were concerns at the rising river levels and "there was concern the river was going to flood".

Waihi Beach Top 10 Holiday Resort owner Ian Smith said the river was breaching its banks in several places and "rising rapidly" when he went to survey the area early this morning.

Smith, a former fire chief of Waihi Beach fire brigade, said: "It was about managing risk, to act sooner rather than later."

Campers were told to pack up their blankets and head to the local RSA, which hosted the evacuees for the night.

Flood waters in some areas of Waihi Beach were knee-deep, Smith said.

He was grateful no one was hurt.

Smith said people were starting to drift back to the campground at about 7.15am and were in good spirits "despite being very wet".

"I just saw a lady now packing her Christmas presents back into the car. They just wanted to have beach Christmas for once but are heading back now.

"A lot of people were hoping to spend Christmas Day here and had their plans thwarted. That's a shame."

Smith said there was talk that there had been 150mls of rainfall in the area since 3pm yesterday.

He was now concerned with a king tide, due to hit Waihi Beach about 8am.

Beachhaven Top 10 Holiday Park manager Simon McCool said there had been constant rain since yesterday and there was a lot of flooding.

"We've flooded like the rest of the beach but our campers aren't affected."

One camper, who had been in a tent, self-evacuated to the RSA and most cars were moved to higher ground, McCool said.

"Most people are still in good spirits. It's not fun though but these things happen."

McCool said the last time Waihi Beach flooded like this was at Queen's Birthday weekend. However, because there was nearby waterway, compared to the other campground, they were at less risk.

Today MetService is predicting a complex low-pressure system to move east over the upper North Island from the northern Tasman Sea, bringing bouts of rain to the North Island tonight through to Christmas Day.

Fronts associated with the low would bring bouts of heavy rain and possible thunderstorms to northern areas of the island.

A heavy rain watch is in place for Waikato, Bay of Plenty and Gisborne from Tolaga Bay northward, MetService said.

Auckland and Great Barrier Island were also under the same watch until 7am Tuesday.

Forecasts for December 25 also looked grim in the North Island.

On Christmas Day, a complex low-pressure system was forecast to move east across the North Island, bringing periods of heavy rain to many places.

"There is high confidence of rain accumulations reaching warning criteria in Northland, Auckland, Coromandel Peninsula, Waikato, Bay of Plenty and northern Gisborne, and moderate confidence in the remaining areas from Taranaki across to northern Hawke's Bay northwards during Christmas day as indicated on the chart," the MetService said.