Tasman Tempest: The anatomy of a storm

The power-packed storm that meteorologists have dubbed the "Tasman Tempest" delivered Auckland more rain than typically falls for the whole of March.

That's according to scientists at the National Institute of Water and Atmospheric Research (Niwa), who have crunched the numbers following the weekend deluge.

Niwa meteorologist Ben Noll said, among other milestones, the weather pattern also produced isolated instances of once-in-a-century rainfall and a tie in the record books for Auckland's wettest March hour.

The tempest, which headed east of New Zealand today, had strengthened off the east coast of Australia a week ago and churned very slowly to the northwest of the North Island over the Tasman Sea for days on end.

"Warmer than average sea surface temperatures east of Australia contributed to the storm's strength and duration," Noll said.

"In fact, the structure of the Tasman Tempest was similar to a tropical cyclone."

Among some of the tempest's most impressive records was the 260mm of rain that Whangamata received in just 24 hours - equal to 156 per cent of the normal March rainfall in one day.

And from last Tuesday to Sunday, Whangamata recorded 475 mm of rainfall, the highest total at any one location from the tempest.

This was 112 per cent of the normal rainfall for the entire autumn season.

According to Niwa's High Intensity Rainfall System, the 225mm that fell in 12 hours over Tuesday night in Upper Hunua exceeded a one-in-100 year event.

Between 3am and 4am on Saturday, Kaitaia had its wettest March hour on record - 44.6 mm - since hourly records began in 1962.

Auckland, according to readings taken at Mangere, tied its wettest March hour on record with 27.6 mm between 5pm and 6pm on Friday since hourly records began in 1965.

Between 9am on Friday and 9am on Saturday, Auckland recorded its wettest March day on record - 100 mm - since 1959.

This was also the third wettest autumn day on record in Auckland.

Hamilton observed its fourth wettest March day on records spanning back to 1907, on Friday with 77mm.

Kaitaia had its 2nd wettest March day on record (since 1948) on Friday with 104mm, putting the Northland town well on track for one of its four wettest March months on record, with 216mm since the beginning of the month - or 267 per cent of the March normal.

Further south, Paraparaumu recorded its second wettest March day on record, since 1951, on Saturday with 74mm.

In the Auckland suburb of New Lynn, where a sink hole opened up during the storm, it was estimated that 60mm fell in two hours on Sunday afternoon, causing flash flooding.

The return frequency on such a rainfall was approximately one in 30 years.

Kumeu recorded 41.4 mm of rain in one hour on Sunday, which was a one in 20 year event according to the High Intensity Rainfall System.

Whangaparaoa was also on track for its wettest or second wettest March on record since 1948, with 226mm having fallen since the start of the month.

That figure was around 317 per cent of the March normal - and 84 per cent of the entire autumn normal.

Noll said while no one weather event is caused by climate change, all events were influenced by climate change since the atmosphere was now warmer and more moist than it was in the past.

"Climate change increases the likelihood of extreme rainfall, given the appropriate weather setup," he said.

"Research suggests that there will be up to 8 per cent more intense rain for every 1C of warming."

In the wake of the storm, high pressure was now forecast to build over New Zealand this week, causing much calmer conditions across the upper North Island and country as a whole.

"The humidity that the Tempest brought will be replaced with typical crisp autumn mornings, though afternoons will be filled with sunshine and comfortable warmth for many," Noll said.

"For those looking to have some late-season beach days, next weekend is looking pretty fabulous across the upper North Island.

"And here's the silver lining: the Tasman Tempest caused some warming of the sea temperatures around the northern portion of the country as warm winds blew from the north for many consecutive days."

Through to the end of the month, high pressure would be favoured to the west and over the country, leading to long stretches of dry weather and sunshine with temperatures near or above average.