Tonga's Attorney-General has quit, saying the government of the Pacific Island kingdom was trying to curb an independent judiciary and attempting to interfere with the inquiry into the Princess Ashika tragedy.

Australian John Cauchi, appointed as the first independent Attorney-General last year, said he was informed yesterday that the Tongan government had taken steps to abolish the Judicial Services Commission and repeal the Judicial Services Commission Act.

"This is the final unconstitutional step required by the Government to destroy the integrity and the independence of the judiciary," he said in a statement.

"It guarantees the future selection of a judiciary which is not independent and beholden to the Government. There appears to be no opposition to this breach of constitutional principles."

Mr Cauchi also said the Government had difficulty in accepting his independent role, and had removed most if not all of his responsibilities.

"These have been allocated to either the Cabinet or the Minister of Justice. This is not the independence the Government said it wanted to achieve when it created the new Attorney-General's position. The Minister of Justice is not the Attorney-General."

Mr Cauchi said the Royal Commission into the MV Princess Ashika tragedy, which sank last year with the loss of 74 lives, had become a benchmark for transparency and openness never seen before in Tonga.

The commission "exposed a level of maladministration and systemic failure" which must be actively addressed by the Government and the Legislative Assembly, he said.

"Instead there has been criticism of the commission and no active response on the part of the Government."

Mr Cauchi said prosecutions arising from the Royal Commission hearings must be allowed to take their course without interference.

"Those who have suffered most through the loss of their loved ones are entitled to expect nothing less."

The Government attempted to have the commission's terms of reference modified before the final report was released. This is not widely known and would be denied, he said.

Two other events had also triggered the decision to resign.

One was the first direct appointment of a judge by the Government without the recommendation of the Attorney General or the Judicial Services Commission.

"This does not comply with acceptable constitutional practice and compromises the integrity of the Judiciary in Tonga."

The other was the decision by the cabinet "not to support" the appointment of independent prosecutors to prosecute matters regarding the sinking of the MV Ashika, "on the spurious grounds of the lack of funding".