The teen, identified by the Australian as 15-year-old Zoe Hosking, was with her stepfather Gavin Dallow on White Island when the volcano erupted on Monday afternoon.
These complications meant the patients needed to be rushed to surgical treatment more urgently that with usual burn cases, Watson said on Wednesday, adding, "This is just the start of a very long process that, for some patients, will take several months".
Jessica Richards (right) with her mother Julie Richards.
Oliver Peterson got the latest from Nine News reporter Lizzie Pearl.
This aerial view shows a group of tourists at the crater's rim before it erupted.
The morning after the eruption, Ngaropo went out with two others at 4 a.m. - the time when he says the connection with the spiritual world is strongest. Many were passengers aboard the Royal Caribbean cruise ship Ovation of the Seas and emerged with burns.
In the town touted as the gateway to White Island, the volcano has an nearly mystical significance, its regular puffing a feature of the landscape.
New Zealand's Deputy Police Commissioner John Tims said yesterday that the police were opening a criminal investigation into the deaths that would accompany an investigation by health and safety regulators.
New Zealand's chief coroner on Wednesday declared the eruption a "mass fatality incident".
The explosion at 2.15pm local time sent a plume of ash and debris 2.2 miles into the sky in one of the deadliest eruptions in New Zealand's history. The patients' burns are already serious from their close proximity to the volcano during the eruption - but the injuries were also complicated by gases and chemicals, Watson said.
Twenty-seven people have horrific burns to 30% or more of their body and 22 are also on airway support due to the severity of their burns, said medical authorities.
Tourist Michael Schade wrote on Twitter as he posted video of the eruption that his "family and I had gotten off it 20 minutes before, were waiting at our boat about to leave when we saw it. Boat ride home tending to people our boat rescued was indescribable".
The first victim to be identified was tour guide Hayden Marshall-Inman, a local of nearby town Whakatane, who has been named by his brother on Facebook.
On Wednesday, an Australian mother and her daughter were confirmed dead by Australian media.
Australia says 13 of its citizens were being treated and 11 were unaccounted for.
The Malaysian High Commission in Wellington said it was informed by New Zealand authorities that at least one out of five immediate fatalities was a Malaysian.
GeoNet raised the alert level for the volcano in November because of an increase in volcanic activity.
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