It could be up to 18 months before the cause of Monday's fire on board PeeJay V is made public, investigators say.

Rob Thompson, chief investigator for the Transport Accident Investigation Commission, said there were three phases to the investigation into what caused the blaze on the White Island Tours vessel.

"The first phase is gathering initial evidence which has started and could take a quite few weeks," he said.

Read more: Passengers took to rubber duckies to escape


Mr Thompson said it was far too early to speculate on the cause of the fire, as there were many different sources of ignition on a vessel of that type and also different fuel source areas.

"We are yet to speak to the passengers and crew whose eye-witness accounts will be integral to our investigation."

Today, PeeJay V crew and coast guard personnel involved in the rescue operation were interviewed by two Transport Accident Investigation Commission investigators.

They also viewed video footage and photographs taken during the rescue operation.

Mr Thompson said it was purely speculation that the cause was an engine room fire. Evidence gathering may include diving on the wreck or salvaging it if possible, he said.

"It really depends what questions are raised from our initial investigation and what value we would get from the wreckage," he said.

"We may be looking at 12 to 18 months before the results of our investigation would be made public," he said.

Read more: Skipper's actions praised for saving passengers' lives

Bay of Plenty Regional Council confirmed no oily debris had been found but there were several areas where charcoaled debris was found.

Debris washed up on the beach from the charter boat was also documented for later assessment.

No decision has been made (or is imminent) on recovering wreckage from the sea floor.

The regional council, with the Department of Conservation and Ngati Awa, will continue to monitor the coastline.

Three locations are under assessment - Coastlands to Whakatane and Rangitaiki river mouths, Otawairere Bay and Ohope beach. Moutohora and Raurima islands would also be checked when the sea conditions were suitable.

"At this stage debris has been located covering an area approximately 400m in length on the Opihi coastlands. This debris is burnt material that has washed ashore," said council spokeswoman Sue-Ellen Craig.

"It is anticipated that the intensity of the fire that consumed the vessel will reduce the likelihood of any environmental effects."

Additional reporting NZME.