WELLINGTON, March 17, 2006 ( Fears mounted Friday for a New Zealand conservation official missing on a remote volcanic island which suddenly erupted, while five other staff were plucked to safety in a dramatic emergency evacuation.

The five took shelter in their staff quarters on Raoul Island while they waited five hours for the rescue helicopter to reach them from New Zealand, 1,000 kilometres (600 miles) away.

They were on the island monitoring the temperature of water in the craters to try to track impending eruptions, but a spokesman for the New Zealand Institute of Geological and Nuclear Sciences (GNS) said the latest eruption "seems to have occurred with no immediate warning".

The missing person was on a routine mission to check water temperatures in a crater lake when the island began erupting at 8:21 am (1921 GMT Thursday).

"Two staff members on the island attempted to search the area where the missing person was thought to be but had to retreat due to further volcanic activity and the track being impassable with fallen trees and ash," the conservation department's area manager Rolien Elliot said.

The pilot of the rescue helicopter reported that about five hectares (12 acres) of native bush had been cleared by the eruption.

Raoul Island, the northernmost outpost of New Zealand settlement, is a volcano and one of the most active sites in the world for earthquakes caused by tectonic plate movement.

Department of Conservation staff, the only people regularly on the nature reserve island -- are trained to take regular readings for GNS to try to track impending eruptions

Raoul is known to have erupted 14 times, the last time in 1964.