Tongan Prime Minister Feleti Sevele and other senior political figures have been cited in a government inquiry into the sinking of a Tongan ferry last August that claimed 74 lives.

The royal commission report blames senior government and state shipping officials, including the Prime Minister, former Transport Minister and a New Zealand head of the state's Shipping Corporation of Polynesia for having contributed to the Princess Ashika disaster.

The report described the incident as "scandalous", and faults failures by officials in buying and operating the vessel that sank in the deep seas on August 5.

Dr Sevele was cited in the report for hiding information, including emails, which showed he played a role in the purchase. "The Prime Minister accepted that there were emails sent to him or by him of direct relevance to the MV Princess Ashika, which he failed to produce in response to the summons served on him," the report said.

"These emails indicate that Dr Sevele played a role in relation to facilitating the purchase of MV Princess Ashika."

The inquiry found the vessel to be overloaded, was unfit for open sea and said government officials knew about it.

The roll-on, roll-off ferry was built in Japan in 1972 for inshore calm water sailing only.

Measuring 50m, it weighed 677 tonnes and was designed to carry about 400 passengers and 15 crew.

As early as 1985, when it was in service in Fiji, the vessel had been classed as unseaworthy.

The vessel did not have an alarm bell or a whistle, and the public address system did not work - so the captain's announcement before the sinking "was heard by few and not comprehended by any".

"The MV Princess Ashika was unquestionably unseaworthy and in an appalling condition. It should never have been allowed to sail in Tonga under any conditions."

The report said Transport Minister Paul Karalus had "absolutely no basis to suggest that the vessel was seaworthy", and if he had carried out his duties properly, "the MV Princess Ashika should never have been purchased".

"The evidence as to the unseaworthiness and appalling state of the vessel is overwhelming and compelling. Mr Sione Mafi Kavaliku, a marine officer in the employ of the Ministry of Transport, when asked by the acting director of Marine and Ports as to his opinion about the vessel aptly responded: 'Any fool [could] tell how bad the ship was'."

A claim by officials "that the vessel was in good condition or well maintained is not only patently absurd, but dishonest," the report said.

The royal commission, chaired by Tongan Supreme Court Judge Warwick Andrew, found the ferry was "grossly overloaded" the day it sank.

The report was handed to King George Tupou V on Wednesday, and the Cabinet will consider the report on Tuesday.

Three people have already been charged with manslaughter in the sinking, and it remains unclear if more criminal charges would be filed following the report.

One of the three, New Zealander John Jonesse, the managing director of the Shipping Corporation of Polynesia, is facing 25 serious charges relating to the disaster, and is set to appear for his preliminary inquiry later this month and was barred from leaving Tonga on Wednesday after a special Supreme Court sitting.

Jonesse, who advised Tongan authorities to buy the ferry, had "clearly misled the board and company secretary as to the condition of the ship. His conduct and actions were deplorable," the report said.

Manslaughter in Tonga carries a maximum prison sentence of 25 years.