Rescuers are also searching for a high school student missing in a flooded cave north of the city.

Heavy rainfall on Tuesday filled basements, stranded cars, toppled trees and disrupted rail services.

The same area was hit by record rainfall in January, and Cyclone Gabrielle a month later.

The torrential rains are expected to last until Wednesday evening local time.

A state of emergency was declared as a precaution and disaster response services have been mobilised, Auckland Deputy Mayor Desley Simpson told reporters.

She described the flooding as an "evolving emergency situation".

Rescue teams were dispatched to Abbey Caves about 2.5 hours drive north of Auckland in search of a missing student, whose classmates made it out of the flood-prone cave safely.

"Our thoughts are with the whanau (family) of the missing child, as well as all those involved in the group outing and the school," police Superintendent Tony Hill said.

From January to April, the Auckland region received 90% of its average total annual rainfall, the New Zealand Meteorological Service's Georgina Griffiths said, with some areas receiving up to 35mm (1.3in) of rain in just one hour.

"The region is very saturated after a very rough five months. We're pretty soggy," she told reporters.

Authorities advised people to postpone unnecessary travel and avoid driving on flooded streets.

Images on local media showed people rushing out of office buildings to go home early, causing traffic jams in some parts of the city.

Cyclone Gabrielle ravaged New Zealand in mid-February, leaving 11 people dead and a trail of devastation that Finance Minister Grant Robertson likened to the damage wrought by the Christchurch earthquake from 12 years ago.

The cyclone hit just weeks after torrential rains and flooding left four dead and parts of Auckland's main airport submerged.