Metservice is warning of thunderstorms and downpours north of Orewa overnight.

Northeasterly winds gusting to 100 km/h in exposed places.

These conditions may lead to additional flooding & slips.

A potentially serious "rainmaker" has moved into northern New Zealand, prompting authorities to declare an emergency as they prepare for another round of heavy rain and gale-force winds.

The "atmospheric river" is already bringing heavy rain and strong winds to parts of the province, and red alerts have been issued for Tauranga and Rotorua.

Lines of thunderstorms are already producing high-intensity rainfalls north of Northland after the region moved into a state of emergency earlier today.

More than 1000 Far North homes lost power this afternoon, with the majority of them north of Kaitaia.

Most customers had power restored by 7:30 p.m., but work to reconnect 113 Waima homes was still underway.

Red heavy rain warnings (highest-level warnings) are in place for Northland, Auckland north of Orewa.

MetService meteorologist Georgina Griffiths is warning of high-intensity rainfall in areas north of Auckland city overnight.

"It's serious," she said.

"The region is just set up ... to flood quickly.

"What we expect is as soon as we get the high intensities on those saturated soils, we will see quick flooding and slips happening."

She said she has not seen rain like Friday's downpour in Auckland before.

"The combination of broadscale rain followed by near record rainfall in that one hour... the widespreadness of those downpours, I've not seen that in my forecasting career."

Auckland Mayor Wayne Brown has urged residents to "stay at home" as much as possible.

Stay safe everyone.