OTTAWA - Parliamentary Budget Officer Kevin Page admits his $30-billion cost estimate to buy and fly the F-35 stealth fighter jets over 30 years isn't as reliable as it could be.

Page told the Commons public accounts committee he was extrapolating from "high-level data" and "broad-based assumptions" to come up with his numbers.

"So, there are always limitations in that sense," he said.

Page testified as the committee studied an auditor general's report that called on the feds to rework their F-35 cost estimates and criticized them for providing inadequate information to Parliament.

Facing aggressive questioning from Conservative MPs, Page also admitted his estimates could benefit from more National Defence data.

"I think these sorts of models should be complemented by much more detailed bottom-up style models to provide real confidence," he said.

National Defence maintains its $25-billion estimate for the F-35, including $10 billion in operating costs over 20 years, is more a reliable number.

Page argues a side-by-side evaluation of differing estimates is like "comparing apples and oranges" because they each include different items over different timelines.

Interim Liberal leader Bob Rae made a surprise appearance at the committee. He took aim at Robert Fonberg, the deputy minister at National Defence, asking why he and the government only talked about a $15-billion estimate publicly, until the auditor general's report unveiled an internal $25-billion figure last month.

"Why was there a $10 billion dollar gap in that discussion?" Rae demanded to know. "Did you authorize the different accounts?"

Fonberg denied that there were any differing accounts, insisting that operating costs don't belong in the acquisition budget.

Outside committee, Page agreed with reporters' suggestions that the federal government deliberately low-balled the F-35.

Then the opposition piled on, accusing the Conservatives of being deceptive.

"They tried to minimize the cost to make it look as if it was a better program and a cheaper program than it was," said the NDP's Malcolm Allen.

Responding to the auditor general's report, the federal government has promised to recalculate its F-35 cost estimates and have them independently verified.