The Conservatives' planned changes to Old Age Security will save Canada anywhere from $10 to $12 billion, Finance Minister Jim Flaherty said Monday.

He let slip the number after a question period in which his colleague, Human Resources Minister Diane Finley, refused to answer questions about the estimate.

Flaherty's officials also refused to provide the estimate at a briefing to parliamentarians at the end of April, although they did give it to reporters covering the federal budget on March 29.

Speaking to reporters after question period, Flaherty allowed that he has heard an estimate of $10 billion.

"I've heard that number, I've heard $12 billion also. Something in that area," he said.

He repeated the government's contention that the cuts are necessary.

"It is very important our largest social program will be available for [Canadians currently under 54 years old] when and if they need that program."

The budget lays out a planned change to OAS that will gradually increase the age at which Canadians are eligible to collect OAS from 65 to 67. The change won't start taking effect until 2023.

Finley wouldn't reveal estimate

Neither Finley, the minister in charge of the OAS file, nor her spokeswoman, would answer a question about what the planned changes are expected to save.

Earlier in the day, Liberal MP Scott Brison said the House was likely to vote on changes to the program before they knew what the government expects to save by increasing the eligibility age.

Finance officials told MPs at a briefing that they would have to wait for a report by the government's chief actuary, Brison said. The chief actuary reports after changes are made to the law, so the report won't be available until after the budget implementation bill becomes law

MPs voted Monday evening on second reading of the budget implementation bill, C-38, which is more than 400 pages long. It includes changes to environmental regulations, OAS, fisheries, immigration and other laws.

Once the bill passes second reading, it moves to the House finance committee. The NDP spent more than a day negotiating with the Conservatives to split the bill into seven parts to make it easier to study, but the government refused, arguing the NDP would vote against it anyway.

Brison pointed to a number of reports, including from the Department of Finance and Parliamentary Budget Officer Kevin Page, that say OAS is already sustainable.

"This is an attack on Canada's most vulnerable. And at a time when Canadians are concerned about income inequality, it's an attack on some of Canada's poorest families," Brison said.

"Until Canadians wake up and understand that they have a government in Ottawa that's trampling democratic freedom... the Conservatives will continue to abuse the system."

NDP finance critic Peggy Nash echoed Brison's concern Monday.

Can't do the math

"The OAS cuts are perhaps the single-most important measure in this budget and they can't even say what it will cost," Nash said.

"How does the minister know that her cuts will make OAS sustainable, when she can't even calculate, can't even do the math to tell us how much these cuts will cost?"

Finley didn't answer the question, instead accusing the NDP of fear-mongering.

"No one who is [currently] receiving OAS or GIS will see cuts," Finley said.

A spokeswoman for the chief actuary said the next required report is expected in about two years and referred questions about any report "reflecting changes currently being debated in Parliament in Bill C-38," to Finley's office.