OTTAWA - Federal Labour Minister Jean-Pierre Blackburn said Monday he can't do his job "by bicycle" but could not explain why his ministerial travel expenses failed to disclose a penny of almost $150,000 worth of charter flights last year.

The Conservatives have made a virtue of their ministerial frugality since taking office, but opposition MPs are now crying foul saying the government is simply hiding such expenses in different books.

Blackburn used the rental planes to tour Quebec as the regional economic development minister for Prime Minister Stephen Harper. But only eight of some 25 flights appeared in his quarterly "proactive disclosure'' of expenses last year, and all eight listed air fare as zero.

Those eight flights alone actually cost taxpayers more than $93,000.

Blackburn's presence on the charters was revealed only through an Access to Information request by the NDP, despite claims by the government Monday that all the information is freely available on the departmental web site.

"When I'm making travel for the department, this spending is on the department (web site),'' Blackburn told reporters. "You could see that over there.''

But the departmental listing shows only the date of the rental contract and its cost -- not who was onboard or where the plane travelled.

And Treasury Board guidelines do not make any distinction between what Blackburn called department business, as opposed to ministerial business.

The posted guidelines under the proactive travel disclosure policy state "the following costs will be charged to a minister's operating budget: official gifts, travel, hospitality, and other services in relation to the department.''

But Blackburn said that when he makes public announcements, the trips are a departmental cost, not a ministerial one.

Clearly anxious to defend his name, he had not been allowed to respond to opposition queries during daily question period in the House of Commons.

Instead, House leader Peter Van Loan parried the opposition attacks by claiming that Liberal ministers of the past spent far more than their Conservative counterparts.

That prompted laughter and incredulity from New Democrat MPs in the House.

"The measuring stick this government is using is how that former government behaved itself? Unbelievable!'' said NDP Leader Jack Layton.

"They campaigned on accountability but they're governing like the Liberals.''

Van Loan shot back that Layton was once outed for using a limousine service while working as a Toronto city councillor seven years ago. At the time, Layton preferred to be photographed riding a bicycle around town.

Liberals outside the Commons said the supposedly frugal Tories are simply skirting the rules.

"They are the ones who adopted all sorts of rules and who keep saying they're absolutely transparent, perfectly accountable,'' said Liberal Marcel Proulx. "They know the rules so they work around them.''

According to Blackburn's officials, the cost of some flights was stated as "$0'' because the rental expense was accounted for elsewhere in government records.

It would be "double dipping'' for Blackburn to have then also recorded those costs in his proactive disclosure, said spokeswoman Emma Welford.

Welford said the disclosure rules may need to be clarified, but Blackburn should not be criticized now.

"The minister would be more than happy to disclose anything in the future if it was required.''