Hot mud and rocks the size of rugby balls shot 60m into the Rotorua sky yesterday when the surface of a park erupted.

A new steam vent, formed as hot water forced its way out of the earth, created a cloud of vapour and mud that killed nearby trees and covered cars in mud in the Rotorua Hospital car park, some 100m away.

Peter Mumby, manager of the commercial laundry based at the hospital, heard a deep rumbling around 3.30 pm and went outside to investigate.   "There was one explosion and the water kept rocketing out for at least two minutes before it stopped."

The hot water was replaced by the steam that is continuing to rise from the ground in Kuirau Park, a geothermal area.

Australian tourist Renee Davies and her family were soaking their feet in the hot pool at the park when they heard what they thought was a helicopter taking off.   "We had been standing taking photos on the rocks in front of [the new eruption] seconds before."

Ranolf St, from Arawa St to Lake Rd, was covered in thick mud, and had to be closed for cleaning before traffic was allowed to use it.

The freshly erected barricades in the park will be in place throughout this weekend.