Captain Doug Monks from New Zealand's Transport Accident Investigation Commission is soon to deliver an interim report on the sinking of the Tongan ferry, Princess Ashika.
His report will be presented in two weeks to the Tongan Royal Commission investigating the sinking, ABC Radio reported.
Capt Monks said a more detailed report will be published in February.
"We've interviewed well over 50 people going into 80 or 90 hours of interviews, we're still processing those interviews," he said.
A total of 72 people died when the ferry sank at midnight on August 5.
So far, 10 people have appeared before the commission, which started this week.
About 30 people, mostly friends and relatives of the victims, along with media, have been attending the hearings.
The Princess Ashika's captain is yet to appear before the commission.
Evidence already presented has included damning photographs of the poor condition of the vessel.
A government marine engineer, Mosese Fakatou, showed 37 photos of the ferry which revealed massive holes, heavily corroded sides and floor and blocked vents that stopped water running off decks.
"There was a photo of a hole on the cargo deck, where one could see down to the ocean," the Matangi Tonga newspaper reported.
"Mosese pointed out that a wide area near the hole was also heavily corroded. He said he had to walk carefully on the whole floor area because he could have fallen through it."
Mr Fakatou, who inspected the MV Princess Ashika to establish seaworthiness the day it set sail on its final voyage, also took photos of worn safety rails and ropes and the entrance gate.
There was also evidence that heavily corroded areas of the boat had been freshly painted over.
That backs up widely-held suspicions among Tongans that the boat, bought from Fiji just six weeks before the tragedy, was not fit for use.
The commission will look at whether any criminal act contributed to the disaster, and whether there's evidence of civil responsibility.
It will hand down a final report at the end of March 2010.
