The Samoan ruling party has appointed a parliamentary committee to scrutinise the conduct of three leading Opposition figures for treason and defamation.

However the leader of the Human Rights Protection Party, Tuilaepa Sailele Malielegaoi, has claimed the government inquiry has stalled over a lack of evidence.

Mr Malielegaoi, along with his deputy leader Fonotoe Lauofo Pierre Meredith and party secretary Lealailepule Rimoni Aiafi, are officially under a parliamentary ethics and privileges investigation.

The three senior men have allegedly made statements accusing Samoan Prime Minister Laaulialemalietoa Leuatea Polata'ivao Schmidt of dishonesty amid acts of criticising the government through parliamentary speeches, media interviews and social media which may breach parliamentary privileges, its standing orders and other laws governing the conduct of MPs.

A spokesperson for Samoa's Fono Legislative Assembly had announced the investigation was set to begin on Monday, however a delay already had Mr Malielegaoi alleging it had been purposely deferred.

The Faatuatua i le Atu Samoa ua Tasi (FAST) Party government have made 200 references where the Opposition was said to have impacted on the "good name" of the Prime Minister and his government since coming to power in September last year.

Mr Malielegaoi has accused the Prime Minister of "pushing" the investigation to deflect untoward attention on his own performance.

"A lot of these accusations are all bullshit," he said at a press conference on Monday.

"Now we'll just wait. The Prime Minister has been too fast in making accusations without any proof."

The investigation has been closely linked to the aftermath of Samoa's 2021 constitutional crisis.

During that period, Mr Malielegaoi and Mr Aiafi initially made public attacks on Samoa's judiciary following a court ruling which enabled a new government to take office after the FAST Party claimed the general election.

Both men were later found guilty of scandalising the court, which under Samoan law is a form of contempt of court, though the court imposed no penalty following their actions.

The parliament later suspended the pair, however the Supreme Court of Samoa ruled the suspension was unconstitutional over procedural defects as the proper legal procedures required before imposing disciplinary sanctions were not followed.

Mr Mallielegaoi, who was Samoa's long-serving Prime Minister from 1998 until 2021, had previously sacked Mr Schmidt from his cabinet in 2020.

Mr Schmidt has since taken over the Prime Minister role from Fiame Naomi Mata'afa last year.

He has repeatedly claimed the three Opposition MPs have made defamatory statements about the government and engaged in "treasonous" conduct following the general election five years ago.

The FAST Party then sought to form a government, however Mr Malielegaoi appeared to be unwilling to hand over power due to a reported combination of constitutional disagreement, legal challenges, and political conviction.

Both the Human Rights Protection Party and the Faʻatuatua i le Atua Samoa ua Tasi (FAST) Party had each won 25 seats, but the parliament's one independent MP, who held the balance of power, later joined the FAST Party to deliver a majority to a new government.

The current investigation has reportedly stemmed from Mr Schmidt moving a motion during parliament's sitting in April where a plethora of allegations with supporting documents were submitted to the parliamentary speaker, Mulipola Aloitalua Mulipola, for a ruling.

Mr Malielegaoi has said attempts from the Opposition to demand in writing confirmation from Mr Mulipola outlining the allegations and the terms of reference had failed.

"We don't really know if the allegations relate to (the political impasse); we keep referring to the lies that have been told ever since that time, and that has irritated the PM," he said.

"I think at the time (of the 2021 general election) it did not seem to matter, but it matters now that he is PM."

The government's press secretariat has declined to respond to the allegations on behalf of the Prime Minister.

During parliamentary debate earlier this year, Mr Malielegaoi allegedly accused the Prime Minister of lying, while Mr Aiafi had allegedly warned the Prime Minister that his term in the top job would be "short-lived".

The parliamentary committee has been given eight terms of reference for the inquiry, including to examine and review how laws dating back to 1960 apply which involve parliamentary powers and its privileges.

The committee is expected to make recommendations on how parliament should respond in its report by October 20.