These thunderstorms are expected to be accompanied by torrential rain.
The rain will ease from the west on Friday evening.
Streams and rivers may rise rapidly, and surface flooding, slips, and difficult driving conditions were possible, the weather agency said.
Rotorua Lakes Council said it was monitoring conditions and on standby.
PowerCO is reporting 621 people have lost power across the North Island this morning, mainly concentrated in the Matamata and Coromandel area.
In the Coromandel Forest Park region, 288 people are affected, and in Matamata, 109 homes are affected.
Bay of Plenty Civil Defence said via its social media page that people should ensure gutters and drains around properties were clear of autumn leaves and tree debris.
“Be sure to also tie down your trampolines or other heavy outdoor furniture as well.”
Most of the country has been put under a severe weather watch or warning this morning.
Slips and flooding are impacting major roads at the top of the South Island, with parts under a local state of emergency.
Auckland and Northland are both under severe thunderstorm watches.
A heavy rain watch is in place for Coromandel Peninsula, Waikato, and Taupō between 9am and 5pm.
The weather is being driven by a low-pressure system in the Tasman Sea, expected to move east of New Zealand tomorrow.
Showers continue for Bay of Plenty and Rotorua on Saturday before becoming mostly fine on Sunday.