And the National Capital Authority insists that no-one has been over-charged for parking in the Parliamentary Triangle, as a result of recent irregularities in credit card payments.

Colourful NSW Senator Bill Heffernan says the impact of paid parking on service workers, who work "bloody hard" at the national parliament, is "horrific".

Senator Heffernan might have been speaking for tens of thousands of public servants and other workers in Canberra's national precincts, who lost their free parking in 2014, when he declared the new regime on Capital Hill was "bullshit", during a Senate Estimates hearing late on Tuesday night.

During his grilling of the authority's chief executive Malcolm Snow, Senator Heffernan gave his frank assessment of the plan to charge for parking under the parliament, which previously offered free parking.

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, political staffers and journalists.

"We have a semi-crisis in the building over parking arrangements and the fact that parking in this building now has a value," Senator Heffernan told Mr Snow.

"The tax office now has a view that parking is a fringe benefit.

"We have people who are cleaners who come here on what you would call average wage and working conditions, who work pretty hard and if we're going to hit them with bloody parking, in my view that is bullshit.

"The cost-benefit analysis may be marginal but the impact financially on people who service this people could be horrific."

The veteran conservative senator also wanted an explanation for the irregular credit card transactions for paid parking revealed by Fairfax Media on Tuesday.

Mr Snow told the committee that a "software upgrade" by the private contractors managing the payments had resulted in historic transactions appearing on motorist's recent credit card statements.

"The complaints that have been received in the last few days have been from people who have received their credit card statements and seen one or more transactions in relation to a parking fee they've paid with their credit card," Mr Snow said.

"In fact those multiple payments date back to an earlier time but the transactions have only just appeared on their statements.

"So no-one is out of pocket Senator ... transactions that were done in October, November and December have only just appeared on people's statements as a result of a software glitch."

"They assure us they have fixed the problem."