The death toll from New Zealand's White Island volcano eruption rose to 18 Sunday, including two people whose bodies have not been recovered, police said.

A land search early Sunday failed to find any sign of the missing pair and divers returned to the sea in the afternoon amid increasing speculation both could be in the water.

Deputy police commissioner Mike Clement said there was "every chance" the bodies had been washed into the sea from the stream where they were last seen Monday.

He added that searchers were "satisfied that the area we searched near the jetty is clear of the bodies".

"The rescue teams are frustrated. We understand completely how frustrating it is for loved ones who want the bodies back," Clement said.

Forty-seven people were on the island -- a popular tourist attraction -- when the explosion happened.

The death toll now stands at 18 after an Australian victim who had been repatriated to Sydney died in hospital almost a week after the deadly eruption.

The family of the latest victim have requested his name and age not be released.

Police on Sunday named seven victims who have been officially identified including New Zealand tour guide Tipene James Te Rangi Ataahua Maangi, 24.

Four were Australians -- Zoe Ella Hosking, 15, her stepfather Gavin Brian Dallow, 53, 51-year-old Anthony James Langford and Karla Michelle Mathews, 32 -- along with Matthew Robert Hollander, 13 and Berend Lawrence Hollander, 16, who were US citizens with Australian permanent residency.

Clement said although the land and sea searches had so far been unsuccessful in finding the remaining bodies, police had not given up hope.

"There will come a time when we've done everything we can do, when we've done everything that's sensible but we're not there yet... we don't give up easily," he said.

Scientists monitoring White Island said there had been no further significant activity since last Monday's eruption but the risk remained with the volcano alert at Level Two, which indicates "moderate to heightened unrest with potential for eruption hazards".

A glow was visible from the vent area overnight "which confirms there is a high heat flow present," said Geoff Kilgour, a volcanologist with GNS Science, which monitors seismic and volcanic activity in New Zealand.

"This has been confirmed today by an aerial observation this morning that noted an active crater is emitting volcanic gas at a high rate and very high temperature" above 200 degrees Celsius (392 degrees Fahrenheit).

The disaster has raised questions about why tourists were allowed on a volcano where experts had recently raised threat levels.