The mayor of Auckland has declared a state of emergency after torrential rain caused widespread flooding and evacuations in New Zealand's largest city.

Auckland Mayor Wayne Brown made the announcement on Friday night as mass evacuations occurred across the city.

"This declaration reflects the extent of damage, displacement and disruption caused by this evening's severe weather event and enables emergency services and response agencies to draw on resources, and enact additional powers, to assist affected Aucklanders," Cr Brown said in a statement.

"The region has experienced widespread damage from flooding and torrential rain, with reports of slips and inundation.

"Infrastructure and emergency services alike have been overwhelmed by the impacts of the storm."

Mr Brown also confirmed that a body had been found at Wairau Valley in the city's north, but police are yet to confirm if the death was flood-related.

Flights into Auckland have been delayed or cancelled due to the wild weather, while footage obtained by the ABC shows areas of the city's airport terminal completely flooded.

"Auckland Airport terminals and roading (sic) are currently impacted by widespread flooding," the airport said on Twitter.

"We are requesting people do not travel to the international terminal at this time."

In a separate tweet, Auckland Airport said runway operations had been reduced after a runway lighting was damaged by a landing aircraft.

Auckland Emergency Management said that northern, north-western and western parts of the city were affected by significant flooding, with conditions expected to worsen.

"Severe weather is currently affecting the Tāmaki Makaurau region, with areas in the north, north-west and west of Auckland affected by significant flooding," it said on Twitter.

"We're working with emergency services to establish what help is needed on the ground."

The agency advised residents in flood-prone areas to prepare to evacuate.

Videos on social media showed firefighters, police and defence force staff rescuing stranded people from flooded homes using ropes and rescue boats.

One video, shot on a busway, showed a packed bus ploughing through water so high that it entered the vehicle and sloshed around passenger's feet.

Major roads were also blocked off by the floods, causing long traffic queues on highways.

Musician Elton John's concert in the city — which was expected to by attended by about 40,000 people — was cancelled.

"Given the weather, tonight's show has been cancelled. Our operations team is working to redirect special event buses to help get people back home … as soon as possible," Auckland Transport said in a statement.

ABC/Reuters