The total cost for Canada's six-month military mission in Iraq could top $166 million, according to a new report from the federal budget watchdog.

That's over $40 million more than the interim estimate released by Defence Minister Jason Kenney on Monday.

A report released by the Parliamentary Budget Office on Tuesday concludes "the estimated incremental cost of Operation IMPACT for six months ranges between a high of $166.40 million and a low of $128.80 million."

Extending the mission for an additional six months could result in a final tab of between $242 million and $351 million.

The report was prepared in response to requests from the New Democrats and the Liberals.

Operation IMPACT

Defence Minister Jason Kenney continues to send signals that Canada's mission in Iraq could be extended and that the fight against ISIS could go beyond Iraq. (Canadian Forces Combat Camera)

Late Monday, Kenney issued a statement pegging the costs to date at "approximately $122 million," a number he indicated would be included in the next batch of estimates, which are expected to be tabled later this week.

In a written statement, 'Kenney was careful to describe the costs as "incremental," and called it "entirely reasonable," given the importance of the mission.

"Canadian Special Operations and our Royal Canadian Air Force are having a meaningful impact against this genocidal terrorist organization, the so-called ‘Islamic State,'" Kenney added.

"Obviously, if we decide to bring our troops back or extend the mission, there would be additional costs and we would seek further approval from Parliament."

Defence refused to turn over info: PBO

Tuesday's report points out that the full costs for the mission in Libya "were almost six times the reported incremental costs."

It also takes the Department of National Defence to task for refusing to turn over information requested by the budget office.

"While DND provided PBO with some information, the majority of PBO’s requests went unfulfilled," the report notes.

"DND made it clear that it took a very narrow view of PBO’s right to access information under the Parliament of Canada Act and would provide information only where it interpreted there to be a clear legal obligation under that act to do so."

Last October, the PBO asked DND to provide its own cost projections for the mission, including the source of funds and a breakdown of how the money would be spent.

According to the report, the department "has repeatedly refused to provide that estimate, claiming it as a cabinet confidence" — an assertion the budget watchdog disputes.

"The estimate is not a cabinet confidence because it provides the factual basis for a decision that has been made, and, regardless, PBO is entitled to the information contained in cabinet confidences provided the information appears in any other document," the report states.

The department also rebuffed a request for "all flying hours to date," apparently "on the grounds that it was not 'financial or economic data,' a claim also challenged by the PBO in the report.

Watchdog barred from briefings

"If 'economic' is to mean anything distinct from 'financial,' it must include factors of production, such as how many employees worked how many hours, or in this case, how many hours have been flown," it argues.

"Such information is essential for producing cost estimates."

PBO officials were also barred from attending technical briefings hosted by the department.

"They were told by DND personnel that such briefings were reserved for stakeholders, i.e., 'academics and analysts who are experts in defence and security.'"

As a result, the PBO "had little option, in many circumstances, but to extrapolate from past missions and operations," which "introduces substantial uncertainty into the analysis."

In a written statement sent out Tuesday afternoon, the defence department said it has and will continue to provide the PBO with information that he needs to do his job, within the mandate Parliament has given him.

"Some of the information requested was deemed to be 'cabinet confidence' and could not be released at that time," the statement said. 

"We remain committed to ensuring that Parliament is informed of the costs associated with the mission which will continue to be reported through the usual parliamentary process."

Liberal MP Joyce Murray, who, along with NDP MP Jack Harris was one of two MPs to request the report, says Canadians deserve "a decent breakdown of the cost" of the mission. Murray noted that the high end of the PBO estimate is 40 per cent more than the projection released by Kenney on Tuesday night.

"The government has been preventing and blocking, that despite the fact that the opposition has been asking for it," she told CBC News on Tuesday.

"National Defence may have violated the Parliament of Canada Act by trying to block access to that information — and as if blocking the PBO isn't enough, the new minister popped up on the eve of the report with no details and no background."

Harris was not immediately available to comment on the report.

But deputy NDP defence critic Élaine Michaud said via email that "from day one," the government has failed to tell Canadians the true cost of the mission.

"Now it’s clear that they may have even gone as far as breaking the law in withholding key information from the PBO,” she added.

“Canadians simply can’t trust the Conservatives to be honest about their approach to this costly operation.”

During question period, NDP Leader Tom Mulcair wondered why Prime Minister Stephen Harper "keeps hiding the truth about his war in Iraq."

In response, Harper said the government has released its estimates, just as it said it would.

"I'm not sure how much clearer it could be," he added.

Last November, then-defence minister Rob Nicholson declined to provide any detail on the cost of the mission.

"[The costs] will be reported, you know, in the normal way," he told MPs on the House defence committee.

"This is a six-month mission, and when the costs are known, we will table them in the usual way."

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With files from CBC News