Department of Conservation officials are mounting an emergency evacuation of Raoul Island, in the Kermadecs, after one of the island's craters erupted today.

One person was missing, the Associated Press news agency reported.

Taupo pilot John Funnell took off at 11am in a MIL17 helicopter, accompanied by a Navajo aircraft, in a bid to reach the dozen DOC staff and volunteer workers on the island.

DOC keeps a boat on the island, 1000km northeast of Auckland, for emergency evacuations.

The island, which is a volcano, began erupting at 8.21am after a strong sequence of earthquakes since Sunday night.

Seismograph data shows this morning's eruption continued for 30 minutes and was at its most intense for about 10 minutes.

It was in one of the volcano's craters, known as Green Lake, and threw out mud, rocks and a plume of steam.

DOC staff monitor the temperature of water in the craters to try to track impending eruptions, but GNS Science said "today's eruption seems to have occurred with no immediate warning".

Raoul Island, last erupted on November 21, 1964 from vents close to Green Lake. That eruption also followed a sequence of strong earthquakes, but also was signalled by big changes in lake level, gas emissions and hot springs. It lasted about half an hour.

A seismic crisis caused by strong earthquakes in March 1993 was not followed by an eruption.

Raoul is a scientifically known island roughly triangular in shape with an area of 29 sq km, around a caldera or big crater 3.3km wide.

A second crater lies in the rim of the cliffs that encircle half of a nearby bay, and a third crater lies east of the main island.

Raoul is known to have erupted 14 times.

- NZPA