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The biggest eruption in Kuirau Park since 2001 has served as another reminder Rotorua is at the mercy of unpredictable geothermal activity.

Steam pressure under a mud pool broke through the surface causing an eruption at 7pm on Sunday. Nobody was in the immediate vicinity at the time.

It lasted about an hour and blew mud 15m high over a 30m radius around the pool, singeing trees and grass.

The area, close to the site of an eruption in 2001, has been draped in a ghostly grey coating of mud.

Crystal, 12, and Daniel West, 9, live on Tarewa Rd near the Rotorua Aquatic Centre and saw the "massive explosion" while they were outside their house.

"It was a big puff. The water was bubbling and there were rocks everywhere," Crystal said.

No one was around when they first went to look at the mess on Sunday night so they went home and came back later with their mum when a small crowd had gathered around the site.

"You couldn't walk around in the mud, your feet would have burnt off," Daniel said.

In January 2001 a major eruption in Kuirau Park left a crater more than 12m wide, destroyed trees and spewed mud and ash up the side of Hospital Hill across the road.

Rotorua District Council geothermal inspector Peter Brownbridge said the eruption was the biggest in the park since 2001. Council staff were at Kuirau Park every day monitoring the pools.

"They look for changes in the spring levels and dying grasses. They're familiar with the area, so they know what is normal and what isn't," he said.

However the staff did not see this eruption coming.

"There are no tell-tale signs. You are at the mercy of the gods on this one. The pressure decides what will happen. It could be one, 10, 100 or 1000 years before it happens again."

He said the only warning staff had had was that the water level in the nearby Lobster Pool bathing area had dropped in the last week.

If someone had been walking past at the time of the eruption they could have been badly burned but there was no reason to worry or avoid the area, Mr Brownbridge said.

He likened the eruption to a can of soft drink being shaken up.

"It's fine until the lid comes off and it squirts everywhere."