Concord Hospital is treating five people, with most in a critical condition, while Royal North Shore Hospital is treating three, two of whom are critical and one of whom is stable.

All of the patients received burns injuries on their bodies ranging from seven per cent to 65 per cent, said Dr Peter Haertsch, Head of the Department of Operating Theatres and Plastic Surgery at Concord Hospital.

The burns were caused by a surge of gases pushed out along the ground at the beginning of the eruption, akin to something seen in the first stages of a nuclear explosion.

"You have patients enveloped by a moving cloud of extremely hot gases and volcanic material," Dr Haertsch said.

After the victims suffered convection burns from the gases, the dust from the eruption settled on their skin resulting in further contact burns.

The victims also inhaled the toxic gases, their respiratory injuries are expected to become apparent in the coming days.

The expert team at Concord Hospital are approaching the injuries as they would any other, Dr Haertsch said.

"It's business as usual as far as we're concerned because we are dealing with cutaneous (skin) burns," he said.

The main priority for doctors now is to remove the burnt, toxic skin from the victims.

Surgeries began on Thursday and are anticipated to wrap up on Friday.

Then the patients will begin a months-long recovery process, including skin graft procedures and painful wound dressings.

"It's a long and demanding business," Dr Haertsch said.

"There will be a lot of surgical and medical papers written about this."

Doctors will study the medical treatment of the White Island Volcano victims for years to come because of the rarity of volcanic burns, he added.

Two more survivors of Monday's volcanic eruption were expected to arrive in NSW from NZ by the weekend.

Australian Associated Press