Canada's budget watchdog has released his own estimates of the Operation IMPACT costs, despite the Department of National Defence's refusal to share relevant data.

According to estimates released by Parliamentary Budget Officer (PBO) Jean-Denis Frechette, at the request of opposition MPs, the price tag for the six-month mission against Islamic State militants in Iraq ranges from a low of $128.8 million to a high of $166.4 million.

On Monday, Defence Minister Jason Kenney said the estimated cost for the full six-month mission, which began last October and will continue to mid-March, is approximately $122 million.

Kenney said the government's own figures, which will be tabled in Parliament this week, could ultimately be higher than his initial estimates.

In his report released Tuesday, the PBO notes his difficulty with accurately estimating the cost, "given the paucity of detailed information available" on a number of factors including the number and type of personnel deployed, the amount of ammunition being used and the Canadian planes' flying hours.

DND refused, he said, "all PBO requests for specific data on Operation IMPACT. Several of these refusals appear to breach DND's legal obligations under the Parliament of Canada Act."

The department argued that details of the Iraq mission are beyond the PBO’s mandate.

"You know, you have to be really narrow-minded, really strict in terms of your interpretation, if you want to always, always say that the data we're asking for is not economic or financial," Frechette said.

"That is why Parliament will have to look at it, and look at the legislation."

Despite the DND "withholding of information", the PBO said his estimates were ultimately based on a combination of costing by analogy and known cost factors.

During question period Tuesday, NDP Leader Tom Mulcair accused Prime Minister Stephen Harper of “hiding from Canadians the truth about his war in Iraq.”

Mulcair said the government had initially vowed Canadian soldiers would not be involved in ground combat, but they ended up engaging in firefights with ISIS. Now, he said, the Conservatives are not being upfront about the true cost of the mission.

Harper said that the government has released the cost “exactly as it said it would,” and accused the NDP of hating “even a cent spent on the Canadian military.”

Harper said his government wants to ensure that personnel engaged in Operation IMPACT are well-paid and well-equipped.

If the mission is extended beyond its current six-month timeline, the PBO says the price tag will rise to between $242.71 million and $351.27 million.

Frechette said he was taken by surprise when Kenney released his department’s estimates for the mission, calling it a case of “bizarre timing.”

"In the morning, you know, the PBO very transparently announces we will publish our report today,” Frechette said. “All of a sudden, in the afternoon, the number was released by the department, by the minister. Good or bad timing, I don't know."