Two more people have died following the volcano eruption in New Zealand, raising the number of people killed to eight.

Both people were being treated in hospital for their injuries when they died, police have said.

Nine others are still missing after the eruption on White Island, but ever-increasing volcanic activity is preventing the authorities from returning.

Emergency services have said the island is too dangerous for a search operation to take place.

Seven Australian nationals are unaccounted for, along with two New Zealanders.

The nationalities of the latest two victims have not been given, but they were among 30 people previously said to have been in hospital after suffering burns following the incident on Monday.

Of those being treated, all but five were in a stable but serious condition.

Due to the unprecedented numbers of burn victims at one time and the nature of their injuries from toxic volcanic gases, more than one million square centimetres of skin has been ordered from America to treat patients.

Dr. Peter Watson, clinical director at Middlemore Hospital, said some patients have burns on up to 95% of their bodies, while the average is 40-50%, and 22 remain on airway support due to burns affecting their lungs.

There were 47 people on the island at the time of the eruption; 24 from Australia, nine from the US and others from countries including Germany, Britain, China and Malaysia, plus five from New Zealand.

On Tuesday, Prime Minister Jacinda Ardern told parliament the "scale of this tragedy is devastating".