The Deloitte audit into expense claims by Senator Pamela Wallin cost $126,998 – or slightly more than the amount she's being asked to repay.

Financial figures published by Senate Administration indicate Ms. Wallin's audit has been the most expensive of four undertaken so far, and roughly tripled in size from its original estimate. The original estimated cost was $43,408, which rose by $25,990 before rising another $57,600 this quarter.

The audit recommends Ms. Wallin pay back $121,348 in travel expenses. Auditors said about three-quarters of that sum was for travel that wasn't ultimately for Senate business, while about one-quarter was for costs incurred by Ms. Wallin during regular stops in Toronto, where she owns a condo. Auditors found no issue with 73 per cent of Ms. Wallin's travel claims.

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The audit into the expenses of Senators Mac Harb, Mike Duffy and Patrick Brazeau cost less – an estimated $101,459.82 in the most recent financial filings, though a Senate spokeswoman said that amount will be updated in the fall. Those three senators were primarily under review for their housing claims.

Ms. Wallin has dismissed her audit process as "fundamentally flawed and unfair" – a notion dismissed by Senator Gerald Comeau, who leads the subcommittee examining her expenses.

"Obviously Senator Wallin has her ideas on the fact she did not like the Deloitte results. We happen to think that Deloitte did a tremendous job. We happen to view them as very professional, highly regarded," he said.

The Wallin expense is the most lucrative contract awarded by Senate Administration in its quarterly report, followed by the audit into the other three senators. All contracts over $10,000 are disclosed under government rules.

Ms. Wallin's audit was released Tuesday, and the Senate committee that received it recommended her case be referred to the RCMP. Mr. Comeau suggested that, in particular, it was inconsistencies in Ms. Wallin's schedule that raised concern – auditors said her calendar had been altered from previous versions.

Ms. Wallin has already repaid $38,369.29 of the $121,348 the Senate is now seeking. The senator has said she "made mistakes" in processing expense claims, and her lawyer stressed in a letter to auditors that it was "important to note, however, that in no case was the Senator seeking personal gain."

Mr. Brazeau, Mr. Duffy, Mr. Harb and Ms. Wallin are all now under investigation by the RCMP. All have stepped away from their party caucuses and are sitting as independents, but continue to serve as senators.