It's believed seven maintenance workers at Alcoa's Kwinana Refinery in Perth have been exposed to asbestos, but the aluminium producer says the health risk is "extremely low".
A contractor noticed a white powdery substance on some insulation tape covering weld seams in a tank roof that was being removed for replacement last month, which testing confirmed was asbestos.
Australian Manufacturing Workers Union state secretary Steve McCartney said people's health should not have been placed at unnecessary risk and called for an immediate audit of asbestos at Alcoa's WA operations.
"Alcoa failed to carry out tests at the site before the workers moved in to do maintenance work," he said.
CFMEU state secretary Mick Buchan said it was an "absolute disgrace" that workers were exposed to asbestos.
"Workers have the right to go to work and do their job without the fear of being exposed to deadly material in the workplace," he said.
Electrical Trades Union state secretary Les McLaughlan said it was not good enough to take short cuts or plead ignorance.
But Alcoa said the company was "deeply concerned" by misinformation and speculation about the asbestos exposure to a small number of contractors.
Director of health and chief medical officer Michael Donoghue said the health risk to the contractors was extremely low due to the non-friable condition of the material, the small amount and type of asbestos, and the short duration of the work being done.
Dr Donoghue said the risk to family and friends of the workers was even lower.
As a precaution, Alcoa has added all similar vessels to the asbestos register.