Anger is increasing among the families of those killed in the Princess Ashika tragedy, as evidence on the disaster comes to light.

A royal commission of inquiry into the August 5 sinking began last week, looking at the circumstances surrounding the disaster and with a particularly close look at the state of the ferry itself.

Up to 74 people - many of them women and children - were killed when the 37-year-old ship sank in Tongan waters late on the night of August 5.

The 54 survivors - all men - said they had only about a minute's warning when the captain announced they were approaching rough waters.

About 11pm, a series of crashing waves smashed into the side of the ferry - causing cargo inside to move to one side - whereupon it lurched abruptly and began to sink.

The women or children were asleep in the passenger cabin below deck.

Just over three months after the tragedy, families of those killed and many in the wider Tongan community are demanding answers and accountability.

Exactly how a rusty and run-down passenger ferry was deemed safe to set sail is what people want to know.

'Ofa Moala, whose husband Sergeant Katalimoni Moala died, says Tongans in the islands and in New Zealand are angry about the circumstances surrounding the disaster.

"You've seen the [Princess Ashika] in the pictures - [the Government] all knew that the boat was rusty and it had holes. They knew about all that and didn't do anything to prevent the accident," Mrs Moala said.

"Why now? What's the use of [the inquiry] now? People had to die for them to act."

Revelations trickling out of the inquiry, such as the fact that the ship had not been inspected fully by a team of people qualified to do so, were "cold", Mrs Moala said.

"They blame the captain and whoever, but they were the ones who bought a bad boat.

"There are so many people to blame - but it all started when they bought the boat. It makes me so angry," she said.

Several top shipping bosses have already been and will continue to be interrogated throughout the inquiry, as the commission tries to piece together how the ferry was bought in its condition and also find out who is to be held accountable for the disaster.

This week, it was revealed that the managing director of the Shipping Corporation of Polynesia, John Jonesse - who bought the Princess Ashika in Fiji on behalf of the Tongan Government - was the only person to inspect the ship before its purchase.

Transport Ministry surveyors were asked to attend the inspection, but did not turn up.