OTTAWA -- Six years after ripping into the Conservative government's cost estimates for F-35 stealth fighters, Parliament's budgetary watchdog says it is digging into the "interim" Liberal plan to purchase Super Hornet jets.

The Liberal government announced last month it wants to buy 18 Super Hornets as a stop-gap measure until a competition can be held in five years to find a replacement for the air force's aging fleet of CF-18 fighter jets.

Prime Minister Justin Trudeau blasted the previous Conservative government for "botching" the replacement of Canada's CF-18s as he defended the plan to purchase Super Hornets before a "genuine, rigorous, open competition."

"We wouldn't have to be going through an interim supplying process if the previous government had actually been able to do the procurement job that they had been tasked with," Trudeau said Monday during a year-end press conference.

"But they were unable to, and therefore we are doing an interim purchase of 18 Super Hornets while we prepare the deep and responsible open competition."

The prime minister glossed over his own government's role in creating a gap between the number of CF-18s ready to fly and how many the air force needs to have ready at any given time by quietly increasing the latter figure earlier this year.

Specifically, the Liberals recently changed a long-standing policy by ordering the air force to be ready to meet Canada's commitments to North American defence and NATO concurrently, rather than managing them together.

The government says the previous policy represented a risk, but critics say the policy change and the decision to buy Hornets are part of a larger Liberal plan to avoid buying the controversial F-35 stealth fighter.

Ministers have admitted that they have an idea how much the Super Hornets will cost and that it will be more expensive in the long run for taxpayers, but they have refused to say by how much.

Experts and internal National Defence reports, including one that was recently removed from the department's website, have warned that operating two different fighter jet fleets would be prohibitively expensive.

While the question of need when it comes to the Super Hornets remains in the air, parliamentary budget officer Jean-Denis Frechette is trying to answer the cost question.

On Dec. 8, Frechette sent a letter to National Defence's top bureaucrat, John Forster, asking for all cost estimates, data and analysis associated with buying and operating the Super Hornets.

The letter, which has been posted to the PBO website, also asks for information on how much more the Super Hornets will cost to maintain and operate than the Royal Canadian Air Force's existing CF-18s.

Defence officials have been asked to respond by Jan. 6, and the PBO will face pressure to complete the study sooner rather than later.

The Department of Public Procurement says discussions are already underway with Boeing and the U.S. government about buying the Super Hornets, though formal negotiations have not yet started.

Peter Weltman, senior director of costing at the parliamentary office, said there is already a great deal of information available with regards to operating Super Hornets in Australia and the U.S.

"So I cross my fingers we'll be able to get it out in the short term," he said. "Several months as opposed to the 10 months it took us to do the F-35."

Frechette's letter provides a familiar echo of a similar request made by his predecessor, Kevin Page, which helped blow the Conservative government's F-35 plan out of the sky in 2011.

In July 2010, the Tories announced plans to buy 65 stealth fighters for $16 billion, but later that year the Commons finance committee asked Page to look into the matter amid doubts about those numbers.

Page's final report, produced despite National Defence's refusal to co-operate, pegged the cost of the jets at $30 billion over 30 years. Critics held it up as proof that the Conservatives lied about the price tag.

Questions about the true costs continued to plague both the F-35 project and the Conservatives until the following year, when auditor general Michael Ferguson's scathing report largely supported Page's findings.

The Conservatives pushed pause on the F-35s in December 2012 after National Defence admitted the costs would actually be more than $45 billion through 2052.

On Monday, U.S. president-elect Donald Trump used his Twitter account to assail the F-35's ballooning costs as "out of control," promising that "billions of dollars can and will be saved on military (and other purchases)" after he is inaugurated Jan. 20.

It wasn't immediately clear what Trump intends to do, though he has repeatedly targeted the stealth fighter program when talking about government waste.

Trudeau, who promised during last year's election campaign that the Liberals would not buy the F-35, described the stealth fighter in June as a plane that "does not work and is far from working."

But the Liberal government has since backed off that promise, saying the F-35 will be allowed to compete in a future competition.