Child Support Agency bosses have told most officials administering payments for 1.2 million children to avoid using its new technology, called Pluto, which quickly hit problems after its launch last year and has been blamed for a blow-out in call wait times.

The tech failure has forced staff to work in both the new and old IT systems since, but most public servants at Human Services this month have returned to working mostly in its aged Cuba payment technology, considered deficient more than five years ago.

Officials are deciding how to reboot the Pluto upgrade project after a $491,000 review by consultants Deloitte, and will let agency staff in Newcastle and Melbourne's new customers teams keep using the technology as others are told to stay out unless necessary.

Staff in other new customers sites will keep using Pluto to register new cases but will complete all other work in Cuba.

Pluto will be available for staff to finish work and review past information, an internal Human Services memo says. Staff still working in the system will help the department test and design it.

The main public sector union said the backtrack was an admission of defeat by the department's management and a new low for Pluto, coming after years of delays trying to fix a system that remained unfit for purpose.

But Human Services said it was still committed to upgrading its child support payment IT with Pluto, and that the decision to move most staff off the technology was a short-term change to ease pressure during a peak period.

The technology is the fourth IT fix to be tried since the then-Labor government announced in 2013 its intentions to replace Cuba, which dates back to 2002.

Community and Public Sector Union deputy secretary Melissa Donnelly said its members working in Human Services' child support division were frustrated by the tech wreck and being told not to use Pluto unless they had to.

“Staff working to help children and families are stuck between a rock and hard place. Cuba is well past its use-by date and Pluto has been a costly failure," she said.

Human Services officials admitted in May it would keep using its obsolete system "for some time yet", and could not say when its replacement would be ready or how much it would cost.

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

The long-awaited new payment system for Australia's child support program ran into trouble almost immediately after its launch, as staff complained it was slower and clumsier than its predecessor and one frustrated operator dubbed the roll-out a "shit show".

A massive, week-long outage crippled the department's child support payment IT earlier this year, an episode the CPSU says shows the system remains vulnerable until Pluto is fixed.

Ms Donnelly said an over-reliance on external contractors rather than in-house expertise was a cause of the problems.

"DHS’s response has been to pay consulting corporation Deloitte hundreds of thousands of dollars to identify problems when they could have just asked their own staff, and now shifting work back to the old system wherever possible.

"The process should now be brought back in-house immediately to ensure there’s the integrity and accountability that’s needed to get things running properly.”