The Conservative senator who heads the Senate's committee of internal economy, the body that monitors senators' expenses, announced Tuesday in the Senate he's resigning as chair for health reasons.

David Tkachuk said he is seeking preventive treatment for cancer, and will leave his post by June 13, although he intends to remain a member of the committee of 15 senators.

"Because that treatment will take me through the summer, there is no guarantee of success," Tkachuk said Tuesday. He added the Senate cannot run the risk of the chair being unable to call a meeting when the chamber is not sitting because he might be ill.

"This is not a decision I came to easily," Tkachuk said. "I don't like to leave jobs unfinished. But at the same time, I don't want to add to the committee's problems."

Tkachuk was chair when the committee received the audits of three senators — Mike Duffy, Mac Harb and Patrick Brazeau — from the private accounting firm Deloitte, and he participated in the removal of critical passages in Duffy's audit to his favour. Those sections were later restored, and Duffy's audit has been sent back to Deloitte for a second look.

Senator Pamela Wallin's expenses are still in the hands of Deloitte, although it's known that she has already repaid almost $40,000 to the Senate.

CBC News has learned that at some point in the months before Deloitte was called in to the Senate in December 2012, the Senate finance director raised concerns to the internal Senate economy committee about Wallin's travel patterns.

At that time, Tkachuk offered to speak to Wallin. It's not known if that meeting took place. However, nothing in Wallin's travel pattern changed after the matter was first raised. The Senate finance director then came back to the committee and said there were still issues, and a decision was made to send the matter to an outside auditor.

Tkachuk announced his resignation from the floor of the Senate, about an hour before the committee he heads met with the auditor general of Canada, Michael Ferguson.

Ferguson was returning to the Senate a year after his report first raised questions about a lack of documentation for some senators' expenses.

The meeting comes a week after the upper house voted to call in the auditor general to conduct a comprehensive audit of Senate expenses.

Auditor general report could take 18 months

Ferguson told the senate committee Tuesday his office must decide whether to examine all senators, although he would certainly take into account the audits that have already been completed. Once finished, he said he would make recommendations where "useful and warranted," and would draw his own conclusions.

He also said even an interim report might take 18 months to complete, although he added that reports could come in different phases, and the Senate can be updated as the work takes place.

Ferguson told reporters waiting outside the meeting room, when asked if he would name names in the audit, "Whether we attach individual names to individual issues — that's not something we've determined yet." He added he'd like to find out whether, if senators say rules around expenses are unclear, if they really are unclear, or if some senators are simply not abiding by them.

Conservative Senator Elizabeth Marshall, a former auditor general of Newfoundland and Labrador, said she believes Ferguson and his staff will have access to any information they seek. "I would expect he would be interested in every nook and cranny," she added.

Marshall also said Ferguson may decide to look at all 105 senators, and says she hasn't heard any senator object to that notion. "I chair the audit committee, and he certainly won't get any resistance from me."

The chamber has been awash in scandal for months over improperly claimed housing, living and travel expenses involving former Conservative senators Mike Duffy, Pamela Wallin and Patrick Brazeau and former Liberal Mac Harb.

Many of the troubles were foreshadowed in a report by Ferguson just a year ago.

In that report, the auditor general found insufficient documentation to determine whether senators had properly claimed a housing allowance or to verify the propriety of some hospitality, travel and living-expense claims.

Liberal Leader Justin Trudeau also wants to involve Ferguson in scrutinizing the other parliamentary chamber.

Trudeau motions defeated

On Tuesday in the House of Commons, he introduced four separate motions designed to shine a light on how MPs spend taxpayers' money. As each one failed, he introduced another:

That travel and hospitality expenses of MPs be posted on a quarterly basis similar to the way cabinet ministers and their staff now list costs of travel to events and any money spent on hospitality.

That MPs' expenditure reports be posted in a form accessible to the public.

That the auditor general do a performance audit of the House of Commons administration every three years.

That a parliamentary committee develop guidelines for the auditor general to perform more detailed audits of parliamentary spending.

All four of Trudeau's motions were defeated because unanimous consent was required. The votes took the form of oral yays or nays. Outside the House of Commons, Treasury Board President Tony Clement said his government and the Conservative caucus supported Trudeau's motions, although at least one reporter listening in the gallery heard nays coming from the Tory side.

Clement said, "Our position is going to be one, that we should in fact as members of parliament have the same proactive disclosure on travel and meals and those kinds of things as cabinet ministers have. And our position is also that the board should be open to the public and to scrutiny."

Once all four of Trudeau's motions were defeated, NDP house leader Nathan Cullen introduced another motion that seemed to be aimed at Trudeau. It asked for an investigation into the "potential" use of members' travel points to see if the points have been used improperly for travel to paid speaking engagements.

Trudeau has been under fire in the past for accepting hefty speaking fees from charities and educational institutions while he was sitting as an MP. He has said that he pre-cleared his speaking jobs with the federal ethics commissioner.

Cullen's motion passed with unanimous consent, meaning that the Liberals voted for it alongside the Conservatives and the NDP.

Speaking to reporters afterwards, Trudeau said, "Not only did it not receive consent from other parties in the House [for his motions], but then they chose to try and play politics by raising an issue of attacking me, that I am more than happy because I have nothing to hide."

Trudeau was also under fire over a weekend television interview, in which he said he'd welcome Liberal Senator Mac Harb back into the Liberal caucus when and if he settles the matter of his housing allowance claims.

Harb has been ordered to reimburse the Senate for $51,500 for wrongly claiming a housing allowance for a supposedly secondary residence in Ottawa, where he has lived for decades and previously served as an MP. Harb has resigned from the Liberal caucus while he prepares to fight the order in court.

Trudeau told Global's The West Block with Tom Clark on Sunday that the Brazeau and Harb cases involved an honest mistake or misunderstanding about the rules and is different from that of the Duffy and Wallin cases.

Heritage Minister James Moore pounced Tuesday on Trudeau's remarks to deflect all Liberal questions about the fact that the prime minister's former chief of staff, Nigel Wright, personally paid Duffy $90,000 to enable him to pay back his invalid expense claims.

Standing in for an absent Prime Minister, Moore accused Trudeau of thinking, "It's perfectly fine for [Harb] to make those expenses, ask taxpayers to pay that money, he thinks it's perfectly OK for him to continue in the Senate."

Trudeau replied, "I've been very clear. If he's innocent, he's in, if he's guilty, he's out. But nobody on this side is going to cut him a $90,000 cheque to avoid the problem."

Harb may yet be ordered to return significantly more than $51,500, which covered only a two-year period. The Senate's internal economy committee has asked for a review of his expenses dating back seven years and, consequently, he could reportedly wind up being asked to repay close to $200,000.