The cost of Wallin's travel — she billed taxpayers $142,190.26 for trips between March 1, 2011 and Feb. 29, 2012 — is perfectly reasonable, Harper said during another barrage of House of Commons questions about Senate conduct.
"Her travel costs are comparable to any parliamentarian travelling from that particular area of the country over that period of time," he said.
"Last year, Sen. Wallin spent almost half of her time in the province she represents in the Senate. The costs are obviously to travel to and from that province, as any similar parliamentarian would do."
Curiously, only $10,551.99 of Wallin's travel expenses were declared as travel between Ottawa and Saskatchewan. The remaining $131,638.27 was filed under "Other."
Conservative MP Garry Breitkreuz, who represents Wallin's home town of Wadena, Sask., charged taxpayers $157,396.67 for travel during roughly that same period.
Travel bills for other Conservative MPs from Saskatchewan ranged from $52,347.09 for Maurice Vellacott to $189,443.59 for Tom Lukiwski.
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Wallin's appointment in 2008 as a senator for Saskatchewan raised eyebrows, since as a former national broadcaster with CTV based in Toronto, she hadn't lived in the province for decades.
She does own property in Saskatchewan, but her travel expenses have cast doubt on how much time she currently spends there, adding her to the list of senators whose residency claims are up for debate.
Wallin told a Saskatoon talk radio show Wednesday that she spent 168 days in the province last year. She said her travel bills are the reality of her commute and the way the Senate expense claim system works.
Most flights back to her home province fly through Toronto, and she often chooses to stay over in the city rather than arrive in Saskatoon in the middle of the night, she told the John Gormley Show.
Wallin maintains a home in Toronto and sits on the board of two Toronto-based companies, Porter Airlines and money management firm Gluskin Sheff.
She also has obligations elsewhere in the country, she noted.
"We do all sorts of other things, not just in Ottawa," she said. "I am an honorary colonel for the air force, I'm in people's ridings, I'm delivering speeches so your travel begins and ends all sorts of different places."
So while she may leave Ottawa and eventually return to Saskatchewan, the flights in the middle don't count as travel to her riding, she said — only direct flights do.
"When you look at the raw statistics, it goes, 'Oh, she travels other places. She doesn't travel to her riding.'"
During question period Wednesday, the Opposition New Democrats continued to rail about whether taxpayers were covering the cost of sending senators like Wallin out for partisan political purposes.
Between Mar. 1, 2011 and May. 31, 2011, when Parliament was dissolved for an election, Wallin billed $25,000 for "other" travel.
"The Senate has a long history of being a dumping ground for party organizers, for party fundraisers, the key fundraisers of parties getting put in there and they continue on that business," NDP MP Charlie Angus said after question period.
"I think with the fact that we have now a number of senators caught up in this scandal of claiming one thing and then not being able to back it up and getting taxpayers money, that’s an issue of trust and it’s an issue of accountability."
Senate travel rules are clear that trips for entirely partisan purposes, such as election activities, cannot be reimbursed.
However, senators can claim "participation in party activities that are related to the work of the senator or the Senate and its proceedings."
Wallin did not immediately return a call Wednesday seeking comment.
Parliamentary rules generally prohibit questions about the Senate from being asked in the House of Commons; the business of the two is supposed to be kept entirely separate.
But senate expenses and ethics have dominated question period in recent days.
On Monday, Sen. Patrick Brazeau, currently on bail, was forced on a leave of absence following charges for assault and sexual assault.
His housing claims — as well as those of Conservative Sen. Mike Duffy and Liberal Sen. Mac Harb — are now under review by an external auditor.
At issue for those two is whether they can claim a housing allowance for maintaining a "secondary" residence in Ottawa, when in fact they appear to live full-time at their Ottawa-area addresses, not the areas they are appointed to represent.
It's unclear whether Wallin's expenses are also being audited.
Wallin told CTV News that she has sat down with an auditor to review her claims, but Senate officials have said her expenses were not referred outside the upper chamber.