Senator Mike Duffy's expenses may get a second review by independent auditors following media reports regarding expenses he claimed while campaigning for Conservative candidates during the last election.

"In light of yesterday's media reports regarding Senator Duffy's expense claims, senators will be asking that the report concerning Senator Duffy be referred back to committee for further examination taking into account this new information," a spokesperson for Marjory LeBreton, leader of the government in the Senate, told the CBC's Hannah Thibedeau in an email Friday.

That move, to determine if Duffy was charging both the Senate and the Conservative Party for his travel expenses, could come as early as Tuesday when the Senate resumes sitting, Thibedeau reported.

Duffy resigned from the Conservative caucus on Thursday evening.

CBC News reported Thursday on one example of the sort of expense records causing suspicion. 

Records filed to Elections Canada by Conservative Joe Oliver's 2011 campaign team in his Toronto riding show an invoice from Duffy as an individual, billing the party for his "campaigning" appearances in eight Greater Toronto Area ridings on April 27 and 28.

The Deloitte report, however, shows Duffy listed as being on "Senate business" on those days, raising the issue of what expenses he may have been charging to taxpayers as a senator at that time.

The NDP on Friday sent a letter to Elections Canada asking it to investigate Duffy's expense claims during the election.

Duffy did not provide all of the information sought by auditors during Deloitte's examination of living and travel expenses. He also did not meet with the auditors to explain his records, although an offer from Duffy to meet with the Senate or Deloitte in late April was turned down by the committee because it did not want to delay the reports.

Both the Deloitte audits and reports from the Senate's internal economy committee were tabled in the Senate and made public on its last sitting day, May 9.

PMO chief's intervention defended

Earlier this week, the Prime Minister's Office confirmed Stephen Harper's chief of staff intervened and wrote a cheque for more than $90,000 to cover the reimbursement of Duffy's expenses.

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Nigel Wright, the prime minister's chief of staff, is facing questions over his decision to write a cheque to pay back more than $90,000 of expenses claimed by Senator Mike Duffy. (Sean Kilpatrick, Canadian Press)

The admission Nigel Wright had written a personal cheque seemed to contradict Duffy's own explanation of his decision to repay his expenses in April. Harper's office said the prime minister was not previously aware of Wright's decision.

On Friday, Harper's spokesman was asked how the prime minister could not have been aware of Wright's decision to write the cheque, which has been characterized as a "gift" and not a loan.

"The overarching concern for the government was that it was clear that the expenses that were claimed [by Duffy] were claimed improperly and had to be returned, and the government was firm in its desire to see those expenses repaid to the taxpayers of Canada," Andrew MacDougall told reporters Friday.

"Obviously, we are now engaged with the office of the ethics commissioner on this so I am not going to add any more comment to that. But from our perspective, the money had to be returned — had to be returned right away. And that was showed when the audits were tabled, that the expenses were improperly claimed and had to be returned, and they had been."

Wright's gift and Duffy's acceptance of it are under investigation by the Commons ethics commissioner and Senate ethics officer, respectively. MacDougall said Friday that Harper's office is "engaged with the ethics commissioner" on the question of Wright's involvement and will co-operate with her investigation. He said any investigation of Duffy is in the hands of the Senate.

MacDougall said again that Wright continues to enjoy the prime minister's confidence. "Mr. Wright is staying on," he told reporters.

Duffy faced mounting pressure

All this hasn't impressed Tory senators, who started an organized effort to force Duffy out of their caucus.

When Duffy resigned from the Conservative caucus Thursday evening, he avoided that push from his colleagues.

When asked whether it was Duffy's own decision to resign from the caucus, MacDougall referred reporters to Duffy's own statement released Thursday, which he said was "accurate as far as I am aware."

Duffy's statement said "it is clear the public controversy surrounding me and the repayment of my Senate expenses has become a significant distraction to my caucus colleagues, and to the government."

Duffy said his presence in the Conservative caucus "only contributes to that distraction," so he will now sit as an Independent. He said he was hopeful he would be able to rejoin the Conservative caucus and looked forward to "all relevant facts being made clear in due course."

Two other senators whose audits were released last week are also now sitting as Independents in the face of their own expenses controversies.

Patrick Brazeau of Quebec and Mac Harb of Ontario are disputing the Senate committee's finding that they need to repay tens of thousands of dollars and are using the audit reports to support legal challenges of the order to repay.

On CBC News Network's Power & Politics Thursday, Brazeau told host Evan Solomon he was "puzzled" by the committee's conclusions, which appeared contrary to the Deloitte audit that "basically concluded that I did not misuse funds."

The RCMP confirmed last weekend that it is reviewing the expenses reports to see if a criminal investigation is warranted.