The Prime Minister's Office had no right to interfere with the audit into Mike Duffy's travel expenses or impose its will on a Senate committee through backdoor channels, the former law clerk of the House of Commons said.

Rob Walsh said it is normal for political considerations to influence the work of Parliament. Still, the government should have let the Senate deal with an administrative issue such as Mr. Duffy's expenses claims by itself, he said, given the independent nature of the audit process.

"The Senate ought to manage its financial affairs independently of the government," Mr. Walsh said on Thursday in an interview with The Globe and Mail.

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He added he was surprised to learn, through newly released court documents, that two Conservative senators made changes to the 2013 report without informing their Liberal counterpart, George Furey, on their audit subcommittee.

"Arguably, this was improper," Mr. Walsh said.

The PMO, as well as the offices of Conservative senators David Tkachuk and Carolyn Stewart Olsen, declined to answer questions on the matter, given Mr. Duffy's ongoing trial on charges of fraud, breach of trust and bribery.

"Questions with respect to a Senate audit are best addressed by the Senate," Conservative parliamentary secretary Paul Calandra said during Question Period. "We have been providing and will continue to provide assistance to the Crown in its case against Mr. Duffy. This matter is before the courts, so it would be inappropriate to comment any further."

The opposition accused the government of having falsified the report into Mr. Duffy's expense claims in an effort to contain the political controversy, making reference to documents that were recently filed in court by Mr. Duffy's legal team.

"This is pure Richard Nixon," NDP Leader Thomas Mulcair said in the House. "The Duffygate coverup was orchestrated right in the Prime Minister's Office. That is what these RCMP court documents prove."

A long-time resident of Ottawa, Mr. Duffy declared after his 2008 appointment to the Senate that his primary residence was his PEI cottage, allowing him to claim expenses for being on "travel status" when he worked in the Senate.

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At the start of the controversy in 2013, the PMO tried to protect Mr. Duffy from questions about his claims, to ensure he would stay quiet until the government found a way to quell the furor.

"A purpose of this is to put Mike in a different bucket and to prevent him from going squirrely in a bunch of weekend panel shows," former PMO chief of staff Nigel Wright said in an e-mail to colleagues on Feb. 7, 2013.

Two weeks later, Mr. Duffy said he would reimburse the expenses. However, he became persona non grata in Conservative circles when it was learned that Mr. Wright ultimately paid back the controversial expenses, totalling $90,000, on his behalf.

Mr. Mulcair pointed to new court documents showing that on May 7, 2013, PMO officials met with Conservative senators to discuss the Senate's audit into Mr. Duffy's expenses. The following day, Mr. Tkachuk and Ms. Stewart Olsen asked the clerk of the Senate to remove some of the precise allegations of wrongdoing involving Mr. Duffy.

"Senator Tkachuk came in here after 4 [p.m.], he wanted to look at this and he reviewed it and he said, 'Carolyn and I would like two more changes,'" former Senate clerk Gary O'Brien told the RCMP.

"That was a confidential report, I'm surprised it was shared so widely," Mr. O'Brien added about the PMO's involvement.

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Mr. Mulcair accused the PMO of "altering and falsifying" the audit, adding that this constituted "a deliberate attempt of obstruction of justice."

Mr. Walsh said that in his view, there are concerns over the actions of Mr. Tkachuk and Ms. Stewart Olsen, as they seemingly made changes to the audit without consulting Mr. Furey.

"If he was left in the dark, then it was inappropriate," the retired law clerk said.