Parliament House cleaners, who are seeking an 85¢ an hour pay rise, could have to pay $11 a day for parking within a month, along with caterers, childcare workers and political staffers.
Senators have grilled Department of Parliamentary Services staff at an estimates hearing over why none of those who might be affected by the move had been consulted about the potential cost.
Labor senator Penny Wong said: "We may have something introduced in a month and we know don't know anything about it."
However, Senate president Stephen Parry, who had known about the issue since July, said it would be premature to consult staff because the parliamentary departments had not yet decided on the best option.
"What if there's going to be no impact on anyone?" Senator Parry said. "There's been a lot of jumping the gun. It may be that not one person pays one cent.
"I'm sure the feeling will be no one wants to pay anything."
He said a lot of contradictory advice had been received, which had delayed the decision-making process.
About 3500 people use Parliament House on a busy day.
The introduction of paid parking would cover a fringe benefits tax liability
The fringe benefits liability crystallised in October when the National Capital Authority introduced paid parking in areas in the parliamentary triangle.
Under tax law, the fringe tax comes into play when paid parking is within one kilometre of a place of employment. Taxpayers may be left to pick up either the fringe benefits or parking bills.
Liberal senator Bill Heffernan said he disagreed with the introduction of paid parking.
"There's lots of ways to save money rather than fiddling with these red ant issues," Senator Heffernan said.
Parliament House is run by several departments.
Senators could pass a motion that staff in the building not be charged for parking and this could reduce the possibility of it being introduced. However, further negotiations would be needed with the Department of the House of Representatives.
The Department of Parliamentary Services will make up to $300,000 annually from its recent introduction of paid parking in the visitors' underground car park at the front of the building.
However, department staff said, overall, it was not a significant revenue measure.
Free parking would be available to anyone who spent $25 at the DPS-owned shop in Parliament.
ACT Labor senator Kate Lundy asked why paid parking was being used to prop up the finances of the shop.