If a volcano's about to erupt in Auckland, scientists will know about it between five and 15 days beforehand, research shows.

"Eruptible magma" could be lurking just below Auckland's surface at any given time – and residents could have as little as five days' warning before a major eruption.

A three-year study, which included shooting laser beams through magma crystals, has helped scientists to make leaps in their understanding of the region's volcanic field.

Until now, estimates for how much warning time Aucklanders would have before an impending eruption sat at a "frustrating" six days to three years, volcanologist Shane Cronin said.

AUCKLAND COUNCIL/SUPPLIED Auckland is built on one of the densest volcanic fields in the world, with more than 50 vents dotted around the city.

"This was not very satisfying for anyone," he said.

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"We are now much more certain about how long it takes for magmas to rise to eruption . . . We are fairly certain that eruptions could occur with only five to 15 days of warning."

SIMON MAUDE/STUFF The research was based on crystal samples from Lake Pupuke.

The findings were vital for informing emergency response systems, Cronin said.

It was estimated that about 435,000 people would need to be evacuated if a volcano threatened to erupt in Auckland.

"Every hour of warning thus would help save lives."

Cronin, an Auckland University professor of volcanology and the director of GNS's Resilience to Nature’s Challenges initiative, said the push to understand Auckland's volcanos was personal, as well as professional.

He and his team were able to make a series of predictions about future eruptions in Auckland by looking at the past eruption of Lake Pupuke on the North Shore.

"I live right next door to Pupuke, and walk . . . around it almost every day, so I have a quite a personal connection to the study site," he said.

"Pupuke is one of the bigger volcanoes in Auckland [and] it's a good comparison for volcanoes like Three Kings, Mt Eden, Mt Wellington, One Tree Hill, Māngere Mountain and Rangitoto."

The scientists took crystals from Lake Pupuke, which were ripped up from the Earth’s mantle in an eruption more than 200,000 years ago, and compared their chemistry to the surrounding magma.

This allowed them to "painstakingly" reconstruct the nature and time span of the eruption.

They found the magma could have been sitting at depths of less than 50km for up one year before the eruption, "so there could be eruptible magma present at any time below Auckland", Cronin said.

The eruption itself was triggered by the arrival of a new, hotter magma batch from deeper in the mantle.

Once the new magma arrived, it destabilised the main magma block and an eruption occurred between 10 and 30 days later.

Cronin said with current seismic detection, "we could assume a warning period of roughly half that – between five and 15 days is likely."

The peer-reviewed findings have been published in one of the top-ranked international geological journals, Earth and Planetary Letters.

Cronin said the results were surprising and exciting.

"We always knew that the Auckland magmas came up quickly – but it was frustrating that we could never say just how quick.

"The work was far more complex and time consuming than I ever thought though, but when we got the results it was pretty exciting.

"It is really satisfying that we have a really good firm for the warning time question in Auckland."