Five survivors of a volcanic eruption on Raoul Island are devastated to have left behind their missing comrade.

Rescuers admitted last night that there was little hope that their Department of Conservation colleague had survived the blast.

The five evacuated workers - two women and three men - arrived at Ardmore Airport by rescue helicopter at 11.40 last night, devastated at having had to leave without finding their colleague.

One rescuer, Senior Constable Barry Shepherd, said: "You can't say with any certainty but I'd say the chances of survival are pretty slim."

Early today, DoC said it was planning to send a search and rescue team to try to find the missing worker, a ranger from the Wellington region.

The eruption destroyed the landscape around the crater where the man was thought to be taking samples.

A rescue helicopter searched briefly but ran out of daylight and a decision was made to evacuate.

Earlier, fallen trees and ash blocked the path of two of the DoC staff searching for their workmate.

His chances of survival looked grim after rescue pilot John Funnell, who landed on the island to take off survivors, estimated that 5ha of bush had been cleared by the 40-second eruption at 8.30am yesterday.

"When you see the devastation that has occurred in the area ... there's ash and mud all over the place," he said.

The eruption from the Green Lake crater threw ash, rock and mud into the air. DoC staff had felt a moderate earthquake estimated at 3 to 4 on the Richter scale 10 minutes earlier.

DoC's Warkworth area manager, Rolien Elliot, said the department was investigating a possible search and rescue mission for the missing man.

Geologists will now work to determine what clues might have pointed towards the eruption.

Clusters of small earthquakes had occurred on Raoul from Sunday to Tuesday, but activity declined after this.

The earthquakes were centred 12 to 15km off Raoul and were related to movement of the tectonic plates.

DoC said there were no warning signs of increased volcanic activity on Raoul itself.

After the eruption, the other five workers took shelter in a safer part of the island and communicated with New Zealand by satellite phone.

A Russian-made MIL17 helicopter was sent from Taupo at 11am yesterday. It refuelled at Ardmore before departing at 1.25pm for the five-hour trip to Raoul, part of the Kermadecs.

Rescue Co-ordination Centre mission co-ordinator Geoff Lunt said the large, twin-engined helicopter could carry up to 30 passengers and crew and was one of only a limited number of helicopters in the country capable of making the round trip to Raoul.

A fixed-wing Navajo aircraft accompanied the helicopter but was unable to land because the Kermadecs do not have an airstrip.

Raoul, at 29.25sq km, is the largest of the Kermadecs and is New Zealand's northernmost piece of land.

Steep and rugged, it lies on the edge of the Kermadec Trench, where the large and active Pacific plate buries itself under the Australasian plate and earthquakes are almost a daily occurrence.

No one lives there except a dedicated DoC team, whose tour of duty begins around October and lasts 12 months, and who are sometimes joined by a team of volunteers, whose main job is pulling weeds.

Apart from their conservancy work, the DoC team gather weather information under contract to the MetService, and monitor volcanic activity for GNS Science by taking temperature readings and checking lake levels.

The challenging lifestyle is compounded by the terrain and duties that can be "repetitive and exhausting".

Michael Rosenberg from the Institute of Geological and Nuclear Sciences, said the eruption was of "a moderate size".

It is the first in more than 40 years - in 1964, workers on the island watched rocks and debris hurled 500m in the air and the island was covered by cloud for days.

One of the station officers at the time, Aucklander Gary Little, yesterday recalled being woken at 6am by a colleague looking out the window.

"He shouted: 'There she goes.' There was a plume of steam and black gunge."

Mr Little said none of the 31 people on the island was "really scared" and some inspected the activity before a naval survey ship arrived after steaming from 100km away.

Three weeks later it was safe for them to return to the island.

Mr Rosenberg said it was "early days" but the latest eruption looked similar to that of 1964.

Back then, however, there were precursors such as a 6m rise in the lake level, and higher temperatures in the ground and hot springs.

He does not know if more eruptions will follow, and leaves for the island himself today to inspect the volcano.

Auckland University head of geology Ian Smith, who has visited Raoul, said the volcano was capable of big eruptions.

"There was a big one 3000 years ago and about 15 since."

He said the DoC hostel was sheltered from the direct effects of the volcano by a substantial ridge unless the eruption became bigger.

Professor Smith said that if the eruption became big enough to send ash high in the air, it could affect air routes to the United States, New Zealand and Australia.