Two more people have died from injuries sustained during the eruption of New Zealand’s White Island volcano, bringing the confirmed death toll from the disaster to eight.

A total of 16 people are believed to have died, including 14 Australian citizens or permanent residents, and New Zealander tour guide, Hayden Marshall-Inman.

The two latest confirmed fatalities were Berend Hollander, 16, and his brother Matthew Hollander, 13, both students at Knox Grammar school in Sydney. Their parents remain unaccounted for.

The Australian Department of Foreign Affairs and Trade confirmed that 28 Australian citizens or permanent residents were caught up in the disaster.

Ten citizens and one permanent resident were presumed dead or missing. This included people who died on the island and whose bodies were brought back to the mainland as well as those thought to still be on the island.

Three people, including two permanent residents and one citizen known to be Coffs Harbour man Jason Griffiths, were brought to hospital but died of their injuries.

Five citizens had left New Zealand for medical evacuation to Australian hospitals. Another seven citizens and one permanent resident were expected to be airlifted later on Thursday.

One citizen is expected to remain at the New Zealand hospital where they are currently being treated.

Dr Ashley Bloomfield, New Zealand’s director-general of health, said there were 21 patients still in New Zealand burns units, including two New Zealanders. They still required “the highest level of care” and a small number were “very unwell”, he said.

The repatriation of Australians in intensive care-capable aircraft “has been going exceptionally well” and had taken pressure off New Zealand units, he said.

“Our intensive care burns teams throughout all of the burns units continue to work around the clock, putting in huge hours and showing, as we know they do, and unbelievable commitment to ensuring the very best of care for patients.”

Bloomfield said they had accepted offers of specialist burns teams to come from US, UK, and Australia.

Australian prime minister Scott Morrison earlier said: “In the days ahead, there will be worse news … I want to thank all of those who have been involved in supporting the families and the friends of those who are missing and presumed deceased.”

The loss of three friends from Coffs Harbour – Karla Matthews, Richard Elzer and Griffiths – was confirmed by their travelling companions late Wednesday night.

“We are incredibly saddened to have lost three of our closest friends,” their friends Alex, Daniel, Ellie, Leanne, Paul and Samantha said in a statement issued by the Department of Foreign Affairs and Trade.

Griffiths died in hospital after suffering burns to 80% of his body in Monday’s eruption while Mathews and her partner Elzer are among eight bodies believed still on the island where authorities say there is no sign of life.

Brisbane mother and daughter Julie and Jessica Richards were also named as victims by their family who remembered them as an adventurous pair who loved the outdoors.

New Zealand police have listed Victorian woman Krystal Browitt as missing on the island.

A total of 16 people are believed to have died from the eruption.

Twenty-eight people remain in hospital, including 25 in critical condition. Many of the injured suffered severe burns and were being treated at hospital burn units around New Zealand.

The volcano vented more steam and mud on Wednesday, prompting authorities to delay plans to recover the bodies of victims.

The family of one Australian who died have questioned whether he would have gone to White Island if he was aware of the risks.

Gavin Dallow, 53, was confirmed dead on Wednesday, while his family said his step-daughter, 15-year-old Zoe Hosking, was presumed dead on White Island.

Jessica and Julie Richards from Brisbane loved have been described as lovers of the outdoors. Photograph: Facebook/AAP

Dallow’s father Brian questioned if his son knew of the risks of visiting the island, home to New Zealand’s most active volcano.

“Gavin was always one for being fairly articulate on what he did,” Brian Dallow said on Wednesday.

“So, I think if he had known there was a danger he wouldn’t have gone on it.

“I’m pretty well sure they weren’t fully informed of the danger.”

New Zealand authorities will examine whether the White Island tour company were aware of any dangers or increased risks of an eruption.

Dallow’s 48-year-old wife and Zoe’s mother, Lisa Dallow, remains in a critical condition in Hamilton with serious burns.

She is one of around a dozen Australians in New Zealand hospitals who the RAAF will try to transfer home for treatment on Thursday.

Many have critical injuries suffered in the intense heat and toxicity of the ash and volcanic gas from the eruption.

Friend John Mickel said the family of Julie Richards, 47, and Jessica, 20, were “united in grief”.

“The family have asked me to describe Julie and Jess as being outdoor, adventurous people ... if there was an adventure that offered itself, then they would go,” he said.

Among the missing are Anthony and Kristine Langford from Sydney and their daughter Winona, 17. Their son Jesse, 19, has reportedly been identified as one of those being treated in hospital.

Police believe 47 visitors were on the island at the time of the eruption, 24 of them Australian, nine Americans, five New Zealanders and others from Germany, Britain, China and Malaysia. Many were passengers aboard the Royal Caribbean cruise ship Ovation of the Seas.