Toll rises to 20 from NZ volcano eruption

Sydney schoolgirl Winona Langford has been officially declared dead

Sydney schoolgirl Winona Langford and a New Zealand man have been officially confirmed dead after last month's volcanic eruption on New Zealand's White Island, bringing the death toll to 20.

The bodies of Ms Langford, 17 and Hayden Marshall-Inman, 40, remain missing with authorities believing they were swept into the surrounding waters.

Ms Langford's parents, Kristine, 46, and Anthony, 51, also died in the eruption while her brother, Jesse Langford, 19, is being treated in Sydney's Royal North Shore hospital.

"The chief coroner has ruled that both Winona and Hayden died on White Island," John Tims, a deputy police commissioner, said in a statement on Thursday.

More than 700 mourners attended a memorial service for Kristine, Anthony and Winona last month at Sydney's Marist College North Shore.

From the official tally of 20 deaths, 18 people died in New Zealand and two in hospitals in Australia.

Survivors of the 47 people, mostly tourists, who were on or near the volcano when it erupted on December 9, are still being treated in hospitals in both countries for severe burns.

Official inquiries into the eruption and New Zealand's response will take up to a year, Prime Minister Jacinda Ardern has said.