A massive eruption in a Rotorua geothermal park threw mud, steam and debris up to 200m into the air on Friday. Up to 100 spectators witnessed the 3.30pm eruption from a mudpool at the central city's Kuirau Park, visible as a huge cloud of steam up to 5km away.

One parked car close to the crater was completely covered in grey mud and small rocks thrown from the erupting pool encircled the area.

One woman said she was walking past the then boiling mud pool when she heard a loud rumble.   She ran across the road and hid under bushes as rocks flew into the air and splintered on the ground.   "I won't be coming back here for a while," she said.   There were no reports of injuries.

Rotorua District Council staff and police cordoned off the active area, although many people took souvenir rocks before the park was completely evacuated.   "We've got quite a lot of rubberneckers trying to have a look at the show," Senior Sergeant Dave Donaldson, of Rotorua police, told the New Zealand Press Association.

Geothermal geologist Ashley Cody said the mudpool was a very active one.   The old pool had a radius of about 2m but the new crater was about 12m, he said.   "I would expect that it would erupt again.   It could erupt many times but it will progressively lose energy."

Institute of Geological and Nuclear Sciences volcanologist Brad Scott told the NZPA an eruption of this size was seen in New Zealand about once every 10 years.   They were often triggered by a drop in air pressure after a dry spell.   He said the eruption might also be linked with a recent swarm of small earthquakes in the Rotorua area.   "It would be difficult to confirm or deny ... but the coincidence in time is very high".   Similar eruptions had been reported in the Kuirau Park area since the 1890s.

Rotorua Civil Defence spokeswoman Carolyn Gordon said police and council staff would work 24 hours to keep the public out of the active area between Kuirau Park and Lake Rd.   "This is part of living in a volatile area and for the safety of the public, please stay away." Barriers have been erected at all the park's entrances.