Some expenses claimed by Senator Pamela Wallin are "very unusual", according to the senior Conservative responsible for Senate spending.

Ms. Wallin is facing an audit of expenses including more than $350,000 in travel over a period of 27 months – an average of nearly $13,000 per month.

The review by Deloitte of Ms. Wallin's expenses, which senators had planned to keep secret, has been ongoing for several weeks. The Senate only confirmed the audit this week and admits it had no plans to make the information public.

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The travel expenses claimed by the senator date from Sept. 1, 2010 to Nov. 30, 2012. Only $29,423.84 of the overall amount is listed as travel between Ottawa and Saskatchewan, which Ms. Wallin represents in the Senate.

Senators must disclose their travel expenses under two categories: Regular travel is defined as a trip between Ottawa and a senator's home province. Other travel is defined as "all other travel," and can include domestic and international trips.

A review of Senate travel expenses by The Globe and Mail shows Ms. Wallin claimed far more "other" travel than any of her senate colleagues since September 2010. Her "other" travel costs of $321,037.58 was well above the second-highest senator in that category: Liberal Senator Colin Kenny, who claimed $192,387.79 during that same period.

In terms of total travel expenses, Ms. Wallin ranks third behind Northwest Territories Liberal Senator Nick Sibbeston and Nova Scotia Liberal Senator Terry Mercer.

Prime Minister Stephen Harper's personal defence of Ms. Wallin was in sharp contrast to the much harder line he has taken in his recent comments related to the three other senators facing expense audits.

He argued in Question Period on Wednesday that her expenses are comparable to the expenses of MPs and senators from similar distances from Ottawa.

"In terms of Senator Wallin, I have looked at the numbers. Her travel costs are comparable to any parliamentarian travelling from that particular area of the country over that period of time," he said.

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Conservative Senator David Tkachuk, who chairs the Senate committee on internal economy and also represents Saskatchewan, said the Wallin audit is the first of what is expected to be regular random audits of senator expenses that are meant to be kept secret unless problems are uncovered.

Yet Mr. Tkachuk's comments to The Globe in an interview also indicate the decision to audit Ms. Wallin was not random.

"There were some unusual items. Nothing illegal, but very unusual, and so that's why we did it," he said. "It has nothing to do with her residency. It has nothing to do with that at all."

Mr. Tkachuk would not specify what he meant by "very unusual," however Ms. Wallin said in a radio interview Wednesday that the way travel expenses are reported makes it look like she travels to Saskatchewan less often than she really does.

She said only plane tickets between Ottawa and Saskatchewan count as "regular" travel and that if she breaks up the trip to Saskatchewan with a stay in Toronto or somewhere else, it falls under "other" travel.

"When you look at the raw statistics, it goes 'Oh, she travels other places. She doesn't travel to her riding,' " Ms. Wallin told Saskatoon's 650 CKOM.

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Mr. Tkachuk had initially denied Ms. Wallin's expenses were being audited when asked directly by The Globe and Mail on Tuesday. On Wednesday, he confirmed the audit and attributed his initial denial to a misunderstanding.

Questions have been raised as to whether Ms. Wallin qualifies as a Saskatchewan resident.

Ms. Wallin's situation is different than the three other Senators who are currently having their expenses reviewed by Deloitte: Patrick Brazeau, Mike Duffy and Mac Harb. All three are claiming a special expense available to senators with a secondary residence in the National Capital Region whose main home is more than 100 kilometres away.

Those three are facing questions as to whether their residence in the Ottawa area is in fact their primary residence, which could mean their expenses would not qualify.

In Ms. Wallin's case, she also claims expenses for a residence in the National Capital Region. However, there are no suggestions that that is her primary residence.

In an opinion piece provided to The Globe, Ms. Wallin wrote that, "My heart is in Wadena, Saskatchewan, and so is my home." She also writes that she spends her summer at a cabin in Fishing Lake, Sask.

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Property records for the Fishing Lake cabin list Ms. Wallin as the owner.

That record also lists her address as a house on Palmerston Blvd., in Toronto. Ms. Wallin is not listed as the owner of that property. She is listed as the sole owner of a condo in Toronto and as one of four owners of a property in Wadena, Sask.

All senators have been asked to prove their current residency status by providing a health card, driver's licence and evidence of where they pay taxes and vote. Ms. Wallin has said she has provided this information, but has not said what it indicates in terms of where she lives.