After draining in two years the entire $102 million budget to replace the system's old IT, the Human Services department could not immediately answer a question at a Senate estimates hearing on Thursday night asking how much the project had cost since depleting the funding in 2015-16.

Centre Alliance senator Rex Patrick said on Friday the project had become a "public money black hole" that appeared to be run in a "barnyard manner" after asking Human Services officials about the Pluto system, which was meant to replace the agency's obsolete technology, called Cuba.

The Department of Human Services is paying consultants Deloitte $491,000 to review its Pluto payment system more than a year after its roll-out was followed quickly by complaints it was slower and clumsier than Cuba.

Human Services secretary Renee Leon told Senator Patrick the plan would become clearer at every round of Senate estimates, held three times a year, but the agency took on notice a question asking how much the project had cost.

"We don't intend to run it in an unbounded fashion," she said.

Senator Patrick said he would write to Human Services Minister Michael Keenan about the Pluto project.

"I'm quite disturbed by the revelation that the secretary is unaware of the amount of taxpayers' money being poured into this project, and has no idea of when it will be complete," he said on Friday.

"It's a public money black hole. It raises the question, how can the secretary meet her statutory obligation to ensure value-for-money if she has no idea on the money being spent?"

Staff have had to use two screens and systems for their work, as the agency keeps Cuba for back-end calculations and payment assurance.

Human Services official Maree Bridger said Deloitte had finished its review, and the agency was considering its findings. The department is forming a "long-term plan" for rolling out Pluto and still wants it to replace Cuba.

Acting chief information officer Charles McHardie said it would use the Deloitte report to determine how long it would keep the old technology.

"Cuba will be with us for some time yet," he said.

Five years ago, Cuba was considered a "deficient capability", which was "unable to fully or easily support Commonwealth government initiatives and directions," according to the department.

Human Services claimed in 2013 that it could have a replacement up and running by December 2015 but more than two years after the mooted launch date for the new system, the agency's child support workers are still using Cuba.

General manager of child support smart centres Bill Volkers confirmed that child support call wait times had blown out at Human Services from three minutes and 18 seconds in the year to March 2017, to seven minutes and 24 seconds in the year to March 2018.

The department denies the roll-out of Pluto is the main cause of its growing child support call wait times, but staff have blamed the system for the blow-out.

Department spokesman Hank Jongen said Ms Leon during Senate estimates on Thursday had refuted claims Human Services had mismanaged the Pluto project.