The family of a Department of Conservation worker feared swept out to sea on remote Raoul Island are distraught as hopes of finding him alive fade.

The Department of Conservation (DoC) volunteer went missing from Fishing Rock on remote the remote island in the Kermadecs about 6am yesterday.

Ground and air searches have failed to find the man, whose name has not yet been released, and the Taupo rescue helicopter has been called in to help.

It arrived at the island this afternoon and was to carry out a 90-minute search this evening of likely areas on the coast, said DoC spokeswoman Liz Maire.

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While it was still an active search, the probability finding the volunteer alive was slim, she said.

"I think hope is dwindling.''

The missing man's family had been notified and were "distraught''.

"They're keeping going until such time as we find him. They're just doing the best they can,'' Ms Maire said.

The man had been due to take water temperature readings for a weather station DoC operates for the MetService.

Some of his measuring equipment was found at the bottom of a walking track down from a cliff where he had parked his four-wheel-drive vehicle, but there was no other sign of him.

DOC spokesman Tim Brandenburg told TVNZ an onboard police search and rescue expert would lead tonight's search.

"We think the likely scenario is that he's put the thermometer in the water, potentially a wave has come up onto the rock and washed him off,'' he said.

Taupo rescue helicopter chief pilot John Funnell said earlier that the chopper would launch its search from Fishing Rock, where the man had gone missing.

"We'll be searching all the little islands, the coastal lines and rocky outcrops to see if we can find the missing party.''

The man had been on Raoul Island, about 1000km northeast of Auckland and renowned for volcanic eruptions and almost daily earthquakes, since October on a six-month stint.

He was one of four volunteers helping three DoC staff who monitor seismic and volcanic activity as well as conduct conservation work such as eradicating weeds to protect the more than 100 plants native to the Kermadecs.

They are the only people on the island, which is a tightly controlled nature reserve.

As a result of the incident DoC planned to review its volunteer scheme.

"We are interested in our staff and our volunteers' safety _ that's the No.1 concern. Obviously, if there is a way of improving that through learning from instances like this, we will implement whatever is necessary. There will be a full investigation,'' Mr Brandenburg told Radio New Zealand News.

DOC worker Mark Kearney, 33, disappeared during a volcanic explosion on the island in 2006. He is believed to have died when a crater lake erupted as he was checking its water temperature. His five colleagues were rescued.