Lead dust has been discovered at the Former Transport Depot in Kingston, the home of the Old Bus Depot Markets.
The hazardous dust was discovered during renovations this month and could have been in the building for many years.
In a statement, arts minister Tara Cheyne said cleaning would take at least three months.
"The Old Bus Depot Markets will not open until after cleaning and remediation is complete," she said.
"Work is still being undertaken to determine the cost and extent of the cleaning and remediation, but it is expected to take at least three months.
"The ACT government will work with contractors, stakeholders and employees who were in the building during construction and may need to undergo testing."
Stallholders were told about the situation today.
The site has been closed since early 2020 while a $6.5 million renovation was being completed to improve safety, accessibility and sustainability of the facility.
Dust samples were collected during the renovation and found to have lead particles. Following that a licensed assessor did surface tests in and around the depot with some of the test result showing a lead reading above the acceptable threshold.
Air monitoring was set up in late January but all test result from that period showed the concentration of atmosphere lead was below detectable limits.
Renovations were due to be completed in March 2021, but the lead discovery will push the reopen date back to at least May.
The ACT government has recently dealt with lead dust contamination in four public schools.
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