Safety inspectors have stopped some renovation work at the Sydney Opera House after asbestos was found by crews working on the $200 million refurbishment.
Electrical workers downed tools on Friday and refused to install cable through the building after the Electrical Trades Union said asbestos had been found for the second time.
SafeWork NSW on Wednesday said scientific testing had confirmed samples taken from woven matting at the site contained friable asbestos.
Builder Laing O'Rourke has been issued a so-called prohibition notice directing it to "conduct a review of safe work procedures in relation to renovation work in the building's ceiling".
Renovation work continues in other areas of the building where it's known that asbestos is not present, a SafeWork NSW spokesman told AAP.
The union says 35 electricians have been exposed to potentially cancer-causing asbestos fibres while installing cabling.
"The NSW government owns this building and must step up to ensure the asbestos removal is done properly as part of this major upgrade," ETU secretary Dave McKinley said in a statement on Wednesday.
Last week's discovery came two months after asbestos was discovered in an accessibility tunnel in the Joan Sutherland Theatre prompting a similar walk-off by workers.