Senator Pamela Wallin charged taxpayers nearly $26,000 in travel expenses during the last federal election period while appearing at a series of Conservative campaign events, according to the NDP.

The Opposition party's ethics critic, Charlie Angus, is demanding to know whether Wallin — the latest senator to become embroiled in expense questions in the appointed upper chamber — used her Senate travel budget for partisan party business.

Wallin's travel expenses, which total more than $350,000 over the past 27 months, are currently the subject of an audit.

The NDP is citing Wallin's 2011 election stops in Collingwood, Ont., and Moose Jaw, Sask., as evidence the former broadcaster was not travelling on Senate business.

"Senator Wallin and other Conservative senators were a very visible presence working for the Conservative party machine," Angus said Thursday outside the House of Commons.

"So what was she (doing) charging that amount of travel expenses ... while she was working on the Conservative federal election? Was that money charged for partisan activities? That's the question that we need to answer."

New Democrats raised the matter during Thursday's daily question period, but it was not directly addressed by the government.

Prime Minister Stephen Harper said all MPs and senators "are fully prepared and committed to have an examination of expenses to ensure that they are appropriate."

Rather than defend or explain Wallin's taxpayer-funded travel during the 2011 campaign, which propelled his Conservatives to a majority government, Harper said her overall travel expenses were comparable to those of New Democrat MPs from western Canada.

Wallin represents Saskatchewan in the Senate, although she hasn't lived there in decades and has a residence in Toronto.

Wallin says she spent 168 days in the province last year.

"These are the costs that parliamentarians incur when they travel back and forth from Ottawa to their provinces," said the prime minister. "That is what the senator has done."

Three others senators — Liberal Mac Harb, Conservative Mike Duffy and former Conservative Patrick Brazeau — are all being audited following questions about their housing expense claims.

According to Wallin's Ottawa office, the senator was not available Thursday afternoon as she was en route to Saskatchewan.