The Senate's internal economy committee has accepted a plan by the federal auditor general to review all senators' expenses, including travel and office expenses, CBC News has learned.

Sources have told CBC News that senators will be required to co-operate.

Auditor General Michael Ferguson was first called to meet with the Senate's committee on internal economy in June, in the wake of the committee's reports on independent audits of expense claims by senators Patrick Brazeau, Mike Duffy and Mac Harb.

That committee issued a report on an audit of Senator Pamela Wallin's travel expenses this week. It recommended she pay back more than $121,000 in expense claims and that restrictions be placed on her travel routes. They are also reviewing a further $20,978 in questionable claims and she could be asked to pay some of them back.

Elizabeth Marshall, who is head of an audit subcommittee, told members of the larger internal economy committee when it met this week to discuss the Wallin report that the auditor general had a comprehensive plan to review all senators' expenses.

Marshall will be the liaison with the auditor general's office to ensure senators and Senate finance staff comply with the audit. Marshall is herself a former provincial auditor general.

CBC News has also learned that members of the committee this week discussed taking steps to put senators on a paid leave of absence, or suspend them from legislative duties such as votes and committee work, if they are under criminal investigation. It is unclear whether this will come before the full Senate when it resumes sitting in the fall.

Staff for audit chosen

(Brazeau was suspended by a vote of the Senate earlier this year, after he was charged with assault and sexual assault. His case is before the courts.)

Ferguson told reporters in June he would have to decide on the scope of a Senate audit and would then discuss an audit plan with the clerk of the Senate and the Senate's audit sub-committee to explain its objectives.

A spokesman for the auditor general confirmed the audit is underway.

"The audit has started. We're in the planning phase," Ghislain Desjardins told CBC News on Friday. "The staff has been identified, the staff has been put in place, so we expect to start soon."

After the June meeting with the Senate committee, Ferguson said a report might take 18 months to complete, although he added that reports could come in different phases, and the Senate could be updated as the work takes place.

His office said on Friday they haven't decided yet if they will release interim reports. Desjardins said they are also still deciding the scope of the audit in terms of how many years of expenses it will examine and he wouldn't comment on whether senators Mike Duffy, Patrick Brazeau and Mac Harb will be included in the review.

"We're not speculating on the scope of the audit as long as the audit is still ongoing," he responded in an email.

When asked what Ferguson would do if irregularities come to light through the audit, Desjardins said, "It's hard to say on what we'll be making recommendations on. We have to wait and see what we will find as the audit unfolds."

Ferguson first raised questions about a lack of documentation for some senators' expenses in June 2012.