The public sector union has taken aim at the government for failing to provide enough trained permanent staff to clear the backlog of robodebt reviews.

Despite evidence to a Senate inquiry that at least 750 workers – half of all compliance staff – have been reassigned to deal with reviews, the Community and Public Sector Union national secretary, Melissa Donnelly, said the government was not delivering the specialist Department of Human Services staff required to clear the backlog.

Two-thirds of the 1,500 compliance workforce are employed through labour hire, which Donnelly said “creates massive churn” in the workforce.

Although there are between 600 and 700 social workers employed by the department, the union believes there may be just 12 senior staff in the specialist integrity and information group who are authorised to sign off on special circumstances debt waivers and conduct vulnerability assessments.

Staff are expected to use an updated operating system manual to identify which debts were calculated in whole or part using only income-averaging – after the government conceded in the federal court that the method was not a lawful basis to raise a debt and abandoned it in a radical program overhaul.

Since early December more than 100 social workers have been given internal authority to work on debt reviews. The union claims that this was done without additional training, although the department says it provided “specific training material to support this change”.

At a Senate committee hearing on Monday, Department of Human Services officials refused to provide any estimate or running total of the unlawful debts that have already been identified out of the total 734,000 robodebts.

Donnelly told Guardian Australia there was “a huge backlog of robodebt reviews that have already been flagged, and the department needs to be provided with more staff to get through this work”.

“Robodebt has been an extraordinary mess, it will take a long time to unpick it, and there’s no way it can get done without proper resourcing and permanent properly trained staff to get the job done.

“The Morrison government robodebt program is putting pressure on an already understaffed workforce. CPSU members are calling on the average staffing cap to be lifted so that they can clean up the mess the Morrison government has created.”

At Monday’s hearing, officials estimated that robodebt reviews would be complete early in 2020.

Ros Baxter, the deputy secretary of integrity and information at the department, confirmed that no extra staff were hired.

Craig Storen, the department’s customer compliance division general manager, said this was because it “had staff available” who were redirected from other activities.

A department spokesperson told Guardian Australia that all 670 social workers were “highly trained to identify and assist people who may be vulnerable and/or have complex circumstances”.

“All of our social workers can offer help to a person who has a debt,” the spokesperson said.

“While some social workers focus on particular programs, for example compliance, child support or carers, it’s important to understand social workers across the department are well qualified to take referrals across any program.”