The Harper government has been handed a lesson in why those who live in glass houses shouldn't throw stones.

Conservatives have been grilled in the Commons on the Senate expenses scandal, and in turn, they've accused opposition parties of ethical lapses of their own — including the fact that two New Democrat MPs have admitted they owe back taxes.

But it's now emerged that one of Stephen Harper's former top advisers also had tax troubles while he was working in the Prime Minister's Office.

Documents filed in court in Gatineau, Que., show that Dimitri Soudas received repeated notices from May 2008 to April 2011 that he owed Revenu Quebec $67,467 in back taxes, including interest.

The matter was finally settled following a court order in March 2012.

Soudas, Harper's former communications director, says the back taxes were the result of clerical errors in filling out his 2008 and 2009 tax forms and — while it took some time to sort out, including a challenge on the interest Revenu Quebec charged — he insists he has never tried to avoid paying his taxes.

"I believe that no tax is a good tax but ... I have always paid my taxes. I have never refused to pay my taxes," he said in an interview Friday.

Soudas said his situation is entirely different from that of NDP MPs Tyrone Benskin and Hoang Mai, who "actually failed to pay their taxes to the point that their salaries may be garnished."

Despite Soudas's trouble, Conservative MPs continued Friday to raise the NDP's alleged tax evasion as a way to deflect opposition questions about Harper's former chief of staff's decision to personally pay the $90,000 tab for invalid expense claims racked up by Sen. Mike Duffy.

They maintained Harper has dealt more swiftly and decisively with the scandal than NDP Leader Tom Mulcair has dealt with the tax evaders in his midst.

"By contrast, the leader of the NDP has been aware of two of his senior critics trying to avoid their taxes," Pierre Poilievre, the government's designated hitter on the Senate scandal, told the Commons.

"One of them (Benskin) is the heritage critic who attempted to introduce a private member's bill that would absolve him from his tax liability. The other (Mai), amazingly, was his revenue critic. That is not the kind of leadership Canadians expect."

Mulcair stripped Benskin and Mai of their critic responsibilities when news of their back taxes broke. Both MPs have promised to pay the money owed.

'Double standard'

The Conservative attacks were blunted, however, by the knowledge that one of their own has also owed back taxes in the past.

"How is this possible when PMO staff have taxes deducted at source?" wondered Liberal MP Hedy Fry.

"Did Mr. Soudas have a side deal like Mr. Duffy to get extra income from the Conservative party? Just what is going on in the PMO?"

Fry got no answers to her questions in the Commons. Soudas later said he had only one source of income during his years working for Harper.

Outside the Commons, opposition MPs accused the Tories of hypocrisy.

"People who live in glass houses shouldn't throw stones," said Liberal MP Marc Garneau.

"And I hope that the Conservatives will be a little more modest in the House of Commons in their attacks against the NDP for not paying their taxes because it's very, very clear that they're not angels themselves."

New Democrat Mathieu Ravignat accused the Conservatives of a double standard that he described as "kind of sad."

"It just underlines the hypocrisy of this government by bringing up taxes when, on their side, it is (also) an issue," he said.

"I think that what we need to do is refocus. The fact is that there's a lack of leadership going on, on this (Senate) issue. We need to do something to clean up the Senate. We need to know this scandal goes all the way into the Prime Minister's Office.

"We need the details."