The Community and Public Sector Union has uncovered "systemic underpayment" at Questacon, with staff only beginning to claw back unpaid wages at the science institution.
Thousands of dollars in backpay are starting to be repaid to hundreds of staff, but the union believes more payment irregularities are yet to be discovered.
The Department of Industry, which oversees Questacon, is yet to quantify the total amount owed to workers.
The unpaid wages were first uncovered in 2023, when reports of underpayment began to surface, with the CPSU initiating a worker consultation campaign that uncovered "widespread cases" of underpayment.
A formal investigation followed.
The probe found staff were not paid their correct overtime allowances and shift allowances as well as other payments, such as the fire warden allowance.
CPSU deputy secretary Beth Vincent-Pietsch said these were now being repaid, but the union believes there may be additional underpayments, and is still investigating.
"Following a 12-month campaign driven by CPSU members in Questacon, the CPSU has secured thousands of dollars in backpay for both current and former employees," she said.
"Questacon required staff to do overtime, take on the responsibility and participate in training to be fire wardens and first-aid officers, and to travel around the country working long and irregular hours. Unfortunately, Questacon failed to correctly pay staff for those duties.
"From our own investigation and work with employees, it is clear to the union that this is a case of systemic underpayment and not a one-off mistake."
A spokesperson for the department apologised for the underpayment and said a new enterprise agreement, that began in April 2024, would address some of these issues.
"The Department of Industry, Science and Resources moved quickly to establish a dedicated working group to resolve the issues promptly," the spokesperson said.
In the 2023 federal budget, Questacon received a $60 million boost over four years, including funding for STEM outreach programs.
