Weather: What caused the deadly Auckland tornado?

Earlier on Saturday, a deadly supposed tornado swept across south Auckland, killing one person, tearing down trees, blowing off roofs and leaving multiple houses uninhabitable.

Meteorologists say the disaster was caused by a low-pressure system driving a rain-band of warm moist air down from the upper North Island where it collided with colder air.

This caused thunderstorms and heavy rain - and then what was likely a tornado formed along this front, causing significant damage in parts of Auckland.

"A band of heavy rain this morning that moved out of Northland has brought mid-winter warmth and a tornado to Auckland," WeatherWatch says.

"Low pressure is creating the unstable weather and while most thunderstorms were offshore from the city the tornado is in line with the very mild weather mixing with cold conditions of winter and low air pressure moving in."

It adds it is possible there was more than one tornado as this front moved down south this morning.

"Tornados are incredibly localised and can come with little to no warning in New Zealand, especially in Auckland with so much coastal influence."

The rain-band has moved south and is now bringing heavy rain to the Coromandel Peninsula and Bay of Plenty, with northern Gisborne in line for heavy rain tonight. MetService says warnings are in place for those areas.

Tomorrow, the low drives easterly winds into central Aotearoa, with heavy rain warnings in place for Wairarapa and the Kaikoura ranges.

There is also a broader area of watches for strong winds and heavy rain from north Canterbury through to Hawke's Bay.