The weather watches and warnings across the North Island have expired but remain in place for the South Island until tomorrow morning.

Multiple flights in and out of Auckland have been delayed and cancelled due to the extreme weather.

Air New Zealand flight NZ5956 from Blenheim was forced to circle off the west of the North Island several times before landing nearly two hours late.

NZ5202 from Blenheim was forced to return to Blenheim, while NZ934L from Tauranga returned home.

NZ5156 from Rotorua and NZ5102 from Palmerston North were cancelled.

A heavy rain warning is in place for North Canterbury and north of the Waimakariri River until 7am.

"Expect 80-110mm of rain about the ranges, and 40-60mm nearer the coast. Peak rates of 10-20 mm/h [are] expected, mainly about the ranges."

A heavy rain watch is also in place for Christchurch, including the Banks Peninsula, until 5am.

For the second day in a row, Aucklanders' phones have pinged with an emergency alert from Auckland Emergency Management, warning of the severe weather.

"Torrential rain can cause surface and/or flash flooding and make driving conditions extremely hazardous," the alert read.

"If your life or property is at threat, call 111."

MetService has issued a red severe thunderstorm warning for parts of Auckland and Waikato as torrential rain, thunder and lightning arrives.

Severe thunderstorms were detected by the MetService weather radar over West Auckland and North Shore suburbs at 6.15pm, and are moving south-southeast.

They are expected to lie over Central Auckland at 6.45pm and rural South Auckland at 7.15pm.

"These thunderstorms are expected to be accompanied by very heavy rain," the forecaster said.

Vector says it has made "substantial progress" in reaching its customers impacted by the weather.

"Our teams continue to monitor the weather closely, and our crews are doing everything they can to reach as many customers as possible, impacted by the weather, today and tonight," Vector said in a 5.30pm update. "For anyone we’re unable to reach tonight, we’re aiming to get to you tomorrow." 

"We’ve made substantial progress today and are now nearing the end of the backlog. 

"We understand how frustrating it is for those final customers still without power, and we truly appreciate your patience as our crews work as quickly and safely as possible to get everyone reconnected. 

"For updates, or to report a fault, visit vector.co.nz/outages.

"As always, remember to stay well away from downed or low-hanging lines, and call 111 if you see any."

Cars on the Northwestern Motorway have slowed to about 50km/hr as rain pelts down. 

Visibility is very poor. Signs along the motorway ask motorists to keep a safe following distance.