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 they needed to evacuate.

Heavy rain warnings are in place for Northland, Auckland, Coromandel Peninsula, Bay of Plenty including Rotorua.