The practice of senators investigating senators is under fire after a committee required three members of the Red Chamber to repay tens of thousands of dollars in expenses but opted not to refer the matter to police.

Opposition politicians expressed outrage over the probe of the controversial expense claims, which was led by Conservative and Liberal senators on the Board of Internal Economy with assistance from independent auditors, because it also appeared that the Conservative committee chair gave his caucus colleague Mike Duffy an opportunity to correct some potentially embarrassing expenses before the final reports were released.

Using Mr. Duffy's cellphone records, auditors reporting to the committee discovered that the PEI senator claimed $87.55 a day over a 12-day period in January, 2012, for working in Ottawa when he was in Florida.

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In a letter, Mr. Duffy wrote that he would repay the $1,050.60 in expenses – which he attributes to an administrative mistake – in April after a conversation with David Tkachuk, the Conservative chair of the board.

Mr. Tkachuk wrote in an e-mail on Thursday that Mr. Duffy had already made the repayment when the conversation took place.

That same week in April, Mr. Duffy announced he had repaid $90,172.24. The committee said on Thursday that the amount was correct and that no more is required. It also ordered Independent Senator Patrick Brazeau to repay $48,744 and Senator Mac Harb to repay $51,482.

Asked why the audits were not referred to police, Marjory LeBreton, the government leader in the Senate, said the rules lack clarity.

"They [the auditors] did in fact acknowledge that some of the rules with regard to expense claims were unclear. They are no longer going to be unclear," she said.

She added that a committee member asked the auditors if they thought further action was required and was told no.

NDP MP Alexandrine Latendresse questioned how everything can be fine as long as the three senators return the money.

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"I think it's clear enough that there seems to be the appearance of fraud and it really demands more investigation," she said on Thursday. "I think it's a matter for the RCMP."

James Cowan, leader of the Senate opposition, said now that the reports are public, police can review them if appropriate.

NDP Leader Thomas Mulcair called the process a "bogus investigation by [the Prime Minister's] handpicked cronies in the Senate."

Ms. LeBreton said Mr. Brazeau and Mr. Harb would be expected to repay the money they owe immediately "or the Senate will take the necessary steps to seize these funds."

Mr. Harb could be on the hook for much more. The initial payback covers a two-year period ending March 31, 2013. The committee has asked for a review of Mr. Harb's expenses for seven previous years.

Mr. Harb, who announced he was quitting the Liberal caucus on Thursday, rejects the findings and has retained former Supreme Court justice Michel Bastarache to challenge the investigation.

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Senate rules allow members to collect as much as $22,000 a year to cover the cost of living in the National Capital Region if their primary residence is more than 100 kilometres away. All three senators claimed their primary residence is outside the 100-kilometre radius, but media reports suggest they spend most of their time at their Ottawa-area homes.

The audits found that while Mr. Harb claimed his principal residence was in Westmeath, Ont., about 140 kilometres west of Ottawa, he was there only about 21 per cent of the time. Mr. Brazeau claimed his principal residence was in Maniwaki, Que. – about 135 kilometres north of Ottawa – yet auditors found he spent only about 10 per cent of his time there. Mr. Duffy claimed his primary residence was in Cavendish, PEI, yet auditors found he was in PEI for 30 per cent of the time and in Ottawa 54 per cent.

They also reveal that Mr. Duffy was not co-operative during the investigation and failed to meet with auditors or provide them with some of the documents they requested.

Conservative Senator Pamela Wallin is facing a separate audit of her travel expenses, which is expected to take longer to resolve.

The expense controversy has contributed to growing calls for reform in the Red Chamber, where senators are appointed on the advice of the prime minister and allowed to sit until their retirement. The government has referred questions about Senate reform to the Supreme Court of Canada.