A monster kauri log hauled out of the ground near Kaikohe could prove immensely important to science.

The log, which is 16m long and weighs 60 tonnes, was found during excavation for a new geothermal power station near Ngāwhā Springs earlier this year.

Last week, scientists completed a radiometric analysis to reveal the kauri stood between 41,000 and 42,500 years ago – making it the only tree found anywhere in the world that was alive during a mysterious shift in the world's magnetic field.

Called the Laschamp geomagnetic anomaly, this event resulted in huge and abrupt changes in atmospheric radiocarbon levels, and the Earth's north and south magnetic poles switching places.

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Associate Professor Alan Hogg, director of Waikato University's Waikato Radiocarbon Dating Laboratory, said it happened to be the most recent and most intensively studied such shift.

"Over a period of 200 to 300 years, the virtual geomagnetic pole moved in clockwise fashion from its original northern position, traveling well into the Southern Hemisphere before swinging north again," he explained.

"If this were to occur at present, it would probably have significant implications for modern technology because very much stronger cosmic radiation impinging on the Earth's surface would almost certainly impact upon satellites and communication."

"We do not need to be alarmist over this but it is important to know just how quickly these changes can occur."

Hogg's team would now be measuring atmospheric radiocarbon levels from 40-ring blocks of wood through much of the tree to establish exactly how radiocarbon changed during its life span.

Another of the reasons scientists were interested in magnetic reversals — and the accompanying drop in the Earth's magnetic field strength, which allowed more solar radiation to reach the Earth's surface — was that they could have a major effect on climate.

"We will be comparing results from dating this tree with dating from other sites to build a clearer picture of past climate changes."

Hogg expected the discovery would be significant to international science.

"This is the only known tree currently found anywhere in the world that spans this important time period."

The tree found at Ngāwhā was part of a vast kauri forest which cloaked the upper North Island before the arrival of humans.

Deforestation accelerated rapidly around 1820 after the arrival of Europeans. It is estimated that only 4 per cent of the original forest remains.

The head engineer of the expansion project, Keith Dickson of Tonkin and Taylor, described the log as a "decent chunk of tree" found about 8m below ground.

"We don't know if there are any more as we have finished everything we wanted to do there."

The tree has been delivered to Ngāwhā Marae, whose chairman Richard Woodman said no decision had yet been made on what to do with it.

"We're just trying to make sure we protect it the best that we can."

- Additional reporting: Peter de Graaf, the Northern Advocate