WHAKATANE (New Zealand) • New Zealand Prime Minister Jacinda Ardern expressed "unfathomable grief" yesterday for tourists caught in a deadly eruption at the White Island volcano, where six people have died and eight more are feared dead.

She held out no hope for the eight people still missing after Monday's tragedy, saying overnight aerial reconnaissance flights had found no signs of survivors.

"The focus this morning is on recovery and ensuring police can do that safely," she told a press conference in Whakatane, a town on the mainland's east coast, about 50km from White Island.

Among the missing and injured are tourists from Australia, the United States, Britain, China and Malaysia, as well as New Zealanders who were acting as guides.

"To those who have lost or are missing family and friends, we share in your unfathomable grief and in your sorrow," Ms Ardern said.

"Your loved ones stood alongside Kiwis who were hosting you here and we grieve with you."

She added: "It's now clear that there were two groups on the island - those who were able to be evacuated and those who were close to the eruption."

In addition to the six dead and eight missing, Ms Ardern said 31 people who were on the island during the cataclysm were in hospital with various injuries, including serious burns.

She said there were legitimate questions to be asked, but they could wait until the emergency response was complete.

"The focus today is on providing critical care for those who have been injured," she said.

Later, in Parliament, Ms Ardern paid tribute to the pilots of four helicopters that landed on White Island in the aftermath of the eruption.

"In their immediate efforts to get people off the island, those pilots made an incredibly brave decision under extremely dangerous circumstances," she said.

Separately, Australian Prime Minister Scott Morrison said yesterday would be a "very hard day" for families of Australians caught up in the deadly eruption. He said the rescue operation had now "moved into recovery phase".

"With 11 Australians unaccounted for, three of those are feared to be among the... deceased," he told reporters in Sydney.

He described the tourist excursion to the volcano as a "time of great innocence and joy interrupted by the horror of that eruption".

Mr Morrison said he was in frequent contact with Ms Ardern, adding that they had feared "very difficult" news and "it is proving to be so".

"This is a very, very hard day for a lot of Australian families whose loved ones have been caught up in this terrible, terrible tragedy," he said.

Thirteen Australians were among those being treated in hospitals across New Zealand, including a number who were in critical condition, Mr Morrison said.

"Obviously, there are quite severe burns issues and that's why they are in so many different hospitals because they have been taken to the various burn units across New Zealand," he added.

AGENCE FRANCE-PRESSE, REUTERS