Explainer - With more heavy rain forecast for the upper North Island this weekend, some are wondering why it has been raining so much and when it is likely to stop.

MetService currently has orange rain warnings in place for Gisborne north of Tolaga Bay and Tasman west of Motueka, and heavy rain watches have been issued for Coromandel Peninsula, Bay of Plenty and Mt Taranaki.

Any forecast rain is coming on top of the havoc wrecked by Cyclone Gabrielle and the Auckland Anniversary Day flood which NIWA described as a one in 200-year event.

How much rain has there been this year?

There was a very wet start to the year with NIWA reporting in January there was 400-800 percent more rainfall than usual for that month in parts of Northland, Auckland, the Coromandel Peninsula, western Bay of Plenty and parts of Hawke's Bay. The highest one day rainfall was recorded in Māngere, Auckland on 27 January when 265mm of rain fell.

Meteorologists say January was Auckland's wettest month since records began and central Auckland experienced more than 45 percent of its yearly rainfall in just one month.

Cyclone Gabrielle hit in February and parts of southern Northland, Auckland, Gisborne, Hawke's Bay, coastal Wairarapa and parts of Canterbury all received at least 400 percent of their normal February rainfall.

The highest one-day of rainfall was recorded at Tūtira Hawke's Bay on 13 February when 316mm of rain fell. On 27-28 February, Gisborne received 51mm and Wairoa 105mm of rain or 77 percent and 119 percent of their monthly normal, respectively, NIWA said.

In March, the situation switched with Northland, Auckland, northern Waikato, coastal Bay of Plenty, Gisborne, northern Wairarapa and Nelson receiving less than 50 percent of their normal rainfall for the month.

By contrast eastern and inland parts of the South Island received more than 200 percent of their usual rainfall and rainfall was also above normal in Wellington, southern Wairarapa, Whanganui and Taranaki. The highest one-day of rainfall was recorded at Mt Cook village on 20 March.

NIWA figures indicate April was quite a dry month for large parts of the country, but that was not the case for Kāpiti Coast and Northland. Paraparaumu on Kāpiti Coast had its wettest April since records began in 1945 with a total rainfall of 202mm of rainfall. Kaikohe in Northland had 371mm of rain, its second highest April rainfall on record.

Sunday 30 April was particularly wet in Kaikohe with 169mm of rain falling - the highest level of one-day rainfall in April since records began in 1956.

Why has it been so wet?

Until March, New Zealand was in a La Niña weather pattern which is associated with northeasterly winds which bring moist, rainy conditions to northeastern areas of the North Island.

La Niña is an atmospheric phenomenon that usually happens every few years, when winds blow warm surface water from the eastern Pacific Ocean towards Indonesia.

NIWA signalled the La Niña pattern was moving away in March 2023 and its principal scientist Chris Brandolino said that would mean less rain for the North Island and perhaps it would be a bit drier in the South Island.

"Because we're transitioning out of La Niña, we still may have these La Niña-like features, so we've got to watch out for the odd downpour," Brandolino said. "But the odds or the chances for getting these successive big rainfall events are certainly going to be declining over the next few months. Still there, but declining."

Atmospheric rivers are massive plumes of moisture that move from the tropics to the mid-latitudes.

NIWA meteorologist Ben Noll said an atmospheric river was one of the factors that contributed to Auckland's deluge on 27 January.

"A slew of environmental factors contributed to this extreme event - a formidable La Niña and marine heatwave led to more moisture being available, which was harnessed by an atmospheric river," Noll said.

"High pressure to the south then blocked it, keeping it in place. The storm was also supported by unique phenomenon called a low-level jet, as well as converging winds that extended lengthwise across the most populated part of the country.

"All of these factors leveraged the atmosphere's tremendous moisture content to drop an entire summer's rain in less than a day. And if it sounds complex, that's because it is - extreme events occur under extreme circumstances."

Training rain or thunderstorms have been another cause of the extremely heavy rainfall experienced in some parts of the country.

NIWA meteorologist Seth Carrier told Stuff that "training" just meant that heavy rain was moving over the same place repeatedly.

Severe thunderstorm warnings have accompanied several of the recent severe weather events with the MetService warning system indicating that heavy rainfall of more than 25mm can result from severe thunderstorms.

A new international study will examine whether the eruption of the underwater Tongan volcano Hunga Tonga-Hunga Ha'apai can be linked to the severe weather events experienced in New Zealand, Stuff reports.

NIWA principal atmosphere and climate scientist Olaf Morgenstern said there was no established link between the events, but it was believed the eruption increased the earth's temperature.

The underwater Hunga Tonga-Hunga Ha'apai volcano sent about 58,000 Olympic-sized swimming pools of seawater into the earth's stratosphere when it erupted in January 2022.

"That water acts like a shield and enhances the green house emissions and enhances the warming," Morgenstern said.

The fact that New Zealand had been deluged with rain this summer was entirely consistent with what could be expected based on global warming, he said.

When is the rain forecast to ease?

NIWA's Seasonal Climate Outlook analysis that was issued on 1 May indicates that El Niño has a 70-80 percent chance of developing during winter and continuing through spring.

NIWA meteorologist Ben Noll said if an El Niño weather pattern developed it would bring southerly winds which were cooler and less moist and brought less rain than the northerly.

NIWA forecasts there could be periods of heavy rainfall in May due to atmospheric rivers, but that conditions were expected to be drier overall in June and July due to a reduction in the amount of tropical and subtropical moisture.

Noll said there could however be a tail-end La Niña effect in May.

"We are moving from La Niña to possibly toward an El Niño, one side of the spectrum to the other, and during that transition period the weather tends to be quite variable," he said.