One of the biggest geothermal eruptions in the central North Island in 50 years could have gone unnoticed had two farm workers not been alerted by a herd of dairy cows.

The big blow, which spewed an estimated 7000cu m of rocks and debris more than 100m, came about 10.30am on Tuesday from a remote rural Department of Conservation reserve halfway between Rotorua and Taupo.

Reporoa sharemilker Phil Morgan was on a farm bike when his herd of more than 500 cows suddenly stampeded up hill and huddled together at the far end of the paddock.

He looked back and saw a huge mushroom cloud heading skyward 100m or more in the air.

Next-door neighbour Dennis Buckley was backing a tractor to load up silage when he, too, glanced over his shoulder and saw the plume of steam rising from a pine plantation several kilometres away across rolling pasture.

He phoned home for his wife Michele to drive up the farm track with the camera.

By late afternoon, the activity appeared to have subsided and the Buckleys tramped into the forest from their Tutukau Rd home to have a look.

"It was what you would expect the moon to look like. All the vegetation was covered with ash and mud," said Mrs Buckley.

The main crater was about 50m wide with a second 20m vent nearby. The ground underfoot was warm.

Rotorua geologist Ashley Cody said after flying over the eruption site yesterday: "In a sea of rich green forest and pasture, there is this off-white to pale grey desolate area of about a hectare. There used to be a mature pine forest with almost impenetrable head-high blackberry in between the trees. Now it's like a firm sandy beach."

About 8km southeast of the Orakei Korako Geyserland Resort, the little-frequented conservation reserve was in an area of hot flowing springs close to forest and alongside pasture, Mr Cody said.

The eruption - more than three times larger than one at Rotorua's Kuirau Park in January 2001 - had thrown boulders and mud over a 100m radius up to 4m thick, he said. Sand and stones (some 10cm in diameter) had spattered on to neighbouring pasture.

Mature trees were felled and denuded, many with the bark ripped off them. "It looks like a huge bomb site."

Mr Cody said there was no evidence of continued eruption yesterday but a pillar of steam was still evident, appearing from a distance to be smoke from a burnoff.

Most locals had been unaware of the thermal blast.

The "most violent phase" on Tuesday morning would have lasted only a minute or two before the eruption reduced, he said.

A similar sized blow had been recorded in the same area in 1948. Mr Cody visits the site every two months to do geothermal monitoring and said he had noticed changes over the past year.

Three big new springs had burst through and started flowing.

"They just suddenly created themselves. We document and measure these things but we don't really know how they will end up."

Mr Cody said he would probably have been unaware of this week's eruption until his next visit in May had the local farmers not been alerted by the spooked cattle.