An $80 billion price tag for Australia's new Attack class submarines - the largest procurement program in the nation's history - went undisclosed for three years.

It also took two years for the government to announce its estimates for the cost of new Hunter class advanced guided missile frigates had risen $10 billion.

The spending came under scrutiny at a Senate estimates hearing on Monday, when Senator Penny Wong asked the department to explain the projected costs.

Defence official Gregory Sammut conceded it took three years for the government to reveal the new submarines would cost $80 billion, after it initially announced the program would involve $50 billion in spending.

He said the two figures resulted from different ways of measuring costs, and that the $80 billion estimate accounted for changes in buying power over the duration of the multi-decade submarine build.

Senator Wong said the broader public would not understand the difference when the government spoke about the cost of submarines.

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It was misleading to describe the cost of a program over time without accounting for changes in the value of Australian dollars, she said.

"No minister, no official, thinks it's reasonable in the largest procurement in the nation's history, to disclose what it will actually cost taxpayers when they're able to do so?" she said.

Media releases from Defence Minister Linda Reynolds as recent as April said the submarines would cost $50 billion.

Senator Reynolds said the public understood the figure didn't include inflation.

"There is nothing confusing in this for the Australian public," she said.

Defence officials also admitted it took two years for the government to announce its estimates for the cost of Australia's new Hunter class frigates had risen $10 billion.

The government said it expected to spend $35 billion when it announced the program, and Senator Reynolds continued referring to the figure until earlier this year.

While the government knew as early as the 2018-19 budget the frigates would cost about $10 billion more, the change wasn't made public until the nation's new defence strategy was released in July.

Senator Wong said there had been a lack of transparency about the costs.

"Essentially the government has known the $35 billion isn't accurate since the 2018-19 budget," Senator Wong said.

"You don't think transparency with people is important?"

Defence Department secretary Greg Moriarty said it was for the government to announce projects and programs.