The search crew looking for a missing Conservation Department worker found no trace of him during today's search.
A long range search helicopter flew from New Zealand to join the search party on Raoul Island today, arriving at 5.30pm. However, it was unable to make progress by the time darkness fell at 8pm.
The missing man is a volunteer who is on the island in the Kermadecs to help with MetService meteorological work.
Conservation Department area manager Tim Brandenburg said while there was still an active and intensive search under way, there were now grave concerns for the volunteer's safety.
"The missing person is a passionate conservationist who was enjoying the opportunity to live and work in such a unique place.
"It was his high level of fitness and previous experience as a ranger that got him a place on the team. We are now very worried that he somehow ended up in the water and was unable to get back to the shore."
The crew had thoroughly covered the island during its search today, Conservation Department spokeswoman Liz Maire said.
"The Rescue Co-ordination Centre has been focussing on the possibility that the person has ended up in the water.
"They have been analysing water flow and the drifts around the island. They have isolated high probability areas and are concentrating on those."
The volunteer's family have requested extra time to come to terms with the situation before releasing his name.
The crew will resume its search tomorrow morning.
Helicopter Services chief executive John Funnell said earlier today RCCNZ were getting the helicopter to concentrate on the area around Fishing Rock, where the man went missing, and on outer rocks and small islands. He may have been washed on one of the rocks or islands.
A pilot and police officer were making the trip, with the rest of the room taken up with fuel tanks.
A Piper Comanche airplane which flew to Raoul yesterday was able to search for just under an hour, mostly concentrated on the water out from Fishing Rock, along with a quick coastal search.
The helicopter would be able to provide the required safety back up, she said.
There were two boats on the island, but for safety reasons before the helicopter arrived both would have needed to be in the water at the same time, and there was only one registered skipper.
A boat would be able to reach areas around the coastline where the ground searchers had not been able to get to, Maire said.
Funnell said he understood that with the two-metre swells along the Raoul coastline yesterday, the searchers had also been apprehensive about launching a boat.
The missing man's vehicle and gear were found close to where he was taking water temperature readings.
DOC Warkworth area manager Tim Brandenburg said that given the currents and conditions on the 2900-hectare island, the ground search was being concentrated along beaches east from Fishing Rock, with beaches to the north also being checked.
As well as finding his vehicle, the thermometer he was using had been in the water at Fishing Rock.
"We think he had begun the work - that appears to be the most likely scenario - that he's been swept out, or fallen into the water somehow," Brandenburg told Radio New Zealand.
Fishing Rock faced to the northeast, and there had been a swell of about two metres at the time the man had been at the spot.
Other DOC workers on the island were distraught, and had been running on adrenalin yesterday.
They were feeling the pressure and keen to continue the search for their missing colleague, Brandenburg said.
Raoul is one of the Kermadec Islands, a nature reserve managed by DOC.
In March 2006, conservation worker Mark Kearney, 33, was killed by an eruption at Green Lake on Raoul Island. His body was never found.