Senators on two powerful committees dealing with Senate expenses were meeting on Parliament Hill Tuesday, including an audit subcommittee that met with federal Auditor General Michael Ferguson, CBC News has learned.

Senator Mike Duffy, who is being investigated by the RCMP for breach of trust, was also seen being driven onto Parliament Hill Tuesday in a silver Mercedes-Benz.

Duffy was one of four senators, along with former Conservatives Pamela Wallin and Patrick Brazeau and former Liberal senator Mac Harb, who were ordered to pay back thousands of dollars in expenses following audits of their claims.

Following a report into Wallin's claims in August, the Senate accepted Ferguson's plan to review all senators' expenses, including travel and office expenses.

The steering committee of the Senate's internal economy committee, which led the probes into four senators' expense claims, met for several hours Tuesday morning. Conservative senators Carolyn Stewart Olsen and Gerald Comeau and Liberal Senator George Furey make up the steering committee.

Later in the day, the separate audit subcommittee met for two hours with Ferguson, the CBC's Julie Van Dusen reported.

Sources told CBC News that Ferguson's audits will begin with senior Senate leaders followed by members of the internal economy committee, before proceeding on to all remaining senators.

Tuesday was the second time the auditor general has met with the audit committee.

"Just a regular meeting, just part of the normal process of doing an audit," Ferguson said on his way in. "I'm not going to talk about the process."

As he left the meeting in the late afternoon, he said, "We're following our normal audit for performance audits — again, all of that rigorous process that we use for performance audits, and that will take us to whatever issues we might find."

Ferguson said he was still putting together the audit team and did not know how many staff will be involved.

"It will be a team of experienced auditors from the [auditor general's] office that will be conducting the work," he said.

Senator Elizabeth Marshall, a former auditor general of Newfoundland and Labrador, sits on the audit subcommittee with fellow Conservative Larry Smith and Furey.

"They've actually started the audit, of course it's still the early stages, but they have started the audit and the purpose of the meeting here today was a progress meeting to bring us up to date as to what's happening," she said after the meeting.

"I've always supported the auditor general coming in to audit the Senate," she said. "I'd like to see him back on a regular basis."

"It's a comprehensive audit and within a comprehensive audit the auditor general and his staff will use the discretion they think is the right method and ways and means to get the type of information that they want," Senator Larry Smith said.

"I think the operative word for all senators is co-operation, and I think you'll find that all senators will co-operate with the auditor general," Smith added.

Furey told CBC News he thinks Canadians "can take great comfort" in the auditor general's work. "The auditor general's going to conduct a comprehensive audit and I think when he's finished people will be relieved."

Comeau said in an earlier interview with CBC News that the committee members were going over the rules for the auditor general's audits. Comeau said he'd had questions from three or four senators about the rules.

At least two Senate administration staffers also met with members of the committees Tuesday: Nicole Proulx, the director of Senate finance, and Senate clerk Gary O'Brien.