Heavy rain is causing havoc in Auckland and Northland as more warnings are issued and people are being warned to prepare for evacuations.

Auckland mayor Wayne Brown has confirmed to RNZ that he has declared a state of emergency and has delegated powers to his deputy, Desley Simpson.

Brown is in Sydney on vacation but will cut short the trip and return to the region tonight.

Auckland Transport says surface flooding is widespread in parts of West Auckland and is urging people to stay clear of floodwaters.

Several schools have also been evacuated in the region.

And an emergency alert has been sent out: "Heavy rain and thunderstorms are affecting the Auckland region and expected to continue into the evening, potentially through to midnight tonight," it said.

It warned to call 111 if life or property was at risk, and to seek shelter with friends and family if you needed to evacuate.

Heavy rain warnings are in place for Northland, Auckland, Coromandel Peninsula, Bay of Plenty including Rotorua and in the South Island for Nelson/Tasman south of Motueka, Marlborough about and north of Awatere Valley and the Grey and Westland districts from Fox Glacier township northward.

Meanwhile near Whangārei, police have confirmed one student is still unaccounted for inside Abbey Caves after a group of 15, along with a teacher and an instructor, became trapped earlier today.

Northland District Commander Tony Hill said the students were exercising at caves but reported being in trouble at about 10.30am.

Whangārei Boys' High School earlier confirmed a group of 15 of its year 11 students were stuck in Abbey Caves, along with a teacher and an instructor.

It said principal Karen Gilbert-Smith had travelled to the caves to support the rescue.

Whangārei District Council said it had been informed that there was an operation underway and that emergency services were leading the response.

The council manages the Abbey Caves but said it could not confirm anything else at this stage.

A resident on Whareora Road which turns onto Abbey Caves said it had been raining all day and it was getting heavy.

Heavy rain warnings are in place for Northland, Auckland, Coromandel Peninsula, Bay of Plenty including Rotorua and in the South Island for Nelson/Tasman south of Motueka, Marlborough about and north of Awatere Valley and the Grey and Westland districts from Fox Glacier township northward.

Main image: A car is caught in floodwaters on Kintara Drive, Massey, as downpours hit Auckland.