A Navy helicopter returns to Whakatane Airport following the recovery operation to return the victims of the December 9 volcano eruption continues off the coast of Whakatane, New Zealand. Picture: Mark Baker/AP

Wellington - The bodies of six people who died when a volcano erupted on New Zealand's White Island earlier this week have been recovered while two are still missing, police said Friday morning. 

"The operation went to plan," Police Commissioner Mike Bush said in Whakatane. "But it's not over yet.

"We do believe that at least one of them is in the water and the other one we are unsure, so that only leaves two possibilities."

Police would continue with aerial searches of the island on Friday afternoon.

New Zealand Defence Force Colonel Rian McKinstry told media a team of six men and two women spent four hours on the island. 

"We cannot downplay the risk involved in this operation," he said. 

The bodies were set to be flown to Auckland to be formally identified. 

Eight others tourists and guides who where on a sightseeing tour at New Zealand's most active volcano when it erupted on Monday were confirmed dead earlier this week.

Mark Inman, right, brother of volcano victim Hayden Marshall-Inman, walks with family members after arriving back to the Whakatane wharf following a blessing at sea ahead of the recovery operation off the coast of Whakatane. Picture: Mark Baker/AP

The Ministry of Health said there were 17 victims still receiving treatment at four hospitals in New Zealand. 

Thirteen Australian patients were taken to burns units in Sydney and Melbourne. 

National geological hazard monitoring system Geonet showed the status of the volcano remained highly volatile. 

Prime Minister Jacinda Ardern said on Tuesday that questions need to be asked after the devastating tragedy. Police and New Zealand's workplace health and safety regulator Worksafe launched separate investigations into the disaster. 

Families of victims of the White Island eruption walk into a nearby marae following a blessing at sea ahead of the recovery operation off the coast of Whakatane. Picture: Mark Baker/AP

White Island sits on the Pacific Ring of Fire, an area known for frequent earthquakes and volcanic eruptions, and is the emergent peak of a large submarine volcano that lies 48 kilometres offshore from Whakatane on the east coast of New Zealand's North Island. 

Tourists have been visiting the privately owned island that is served by several operators for more than 30 years.

While seismic unrest was frequent, this was the first deadly eruption since a mudflow in the wake of a volcanic breakout killed 10 sulphur miners in 1914.