The death toll from last month’s volcanic eruption in New Zealand has grown to 18 after one of the injured died in a hospital in Australia, police say, while two remain missing and are presumed dead.

“Police can confirm a further person died in an Australian hospital last night as a result of injuries suffered in the Whakaari/White Island eruption,” New Zealand’s deputy police commissioner, John Tims, said in a statement on Monday.

Police did not provide details on the identity or nationality of the person. The majority of the previously named victims after the powerful eruption of White Island, also known by its Maori name of Whakaari, were Australian citizens or permanent residents.

Two others are still missing since the incident and presumed dead. New Zealand has scaled down the search operations and has acknowledged that the bodies of the two missing people may never be found.

There were 47 people, mostly tourists, on or near the White Island volcano when it erupted on 9 December. Those who survived are still being treated in hospitals in New Zealand and Australia for severe burn injuries.

Official inquiries into the eruption and New Zealand’s response will take up to a year to conclude, prime minister Jacinda Ardern has said.