Electricians working on $200 million Sydney Opera House renovations have walked off the job following the discovery of asbestos.

About 35 workers stopped part way through installing cabling at the iconic Australian landmark after asbestos was found in work areas.

The stoppage comes just eight weeks after asbestos was found in the building’s wiring, which the Electrical Trades Union (ETU) says led to the exposure of 25 workers working in a service duct.

“This issue was first identified two months ago, with SafeWork NSW issuing improvement notices to builder Laing O'Rourke giving the company seven days to remove the asbestos or eliminate the threat to workers through appropriate safety measures,” ETU secretary Dave McKinley said.

“Electricians yesterday raised the alarm that they were again being exposed to loose asbestos fibres, which has now been confirmed by scientific testing.

“Two months after this major safety issue was uncovered, and the builder was ordered to rectify it by the safety regulator, we have again seen workers exposed to these carcinogenic fibres.”

Mr McKinley said the exposure may not affect workers for decades.

“These guys won't know for 15, 20 maybe 30 years if this exposure is actually fatal,” he said.

“Throughout my career I've worked with people who have died from asbestos and it's a horrible, horrible miserable disease.”

A spokesperson for the contractor on the $200 million renovation - Laing O'Rourke - confirmed two “unexpected” asbestos finds.

“Safety is always a top organisational priority for both Laing O'Rourke and the Sydney Opera House,” the company said.

“As with many Sydney buildings constructed in the 1960s and 1970s, the Opera House contains asbestos.

“We have planned for the management of asbestos in known and unknown locations during the course of the Joan Sutherland Theatre upgrades and there is a comprehensive Asbestos Management Plan in place.”

The ETU wants NSW Better Regulation Minister Matt Keen to launch an investigation into why SafeWork has not handed Laing O'Rourke a prohibition notice.

The notice would prevent work from taking place until the asbestos is isolated and removed.

“It is completely unacceptable that workers, performers and the general public continue to be exposed to a toxic substance at this iconic building,” Mr McKinley said.

“Particularly as the builder is receiving $200 million from taxpayers to carry out the renovations.”

Laing O'Rourke reportedly threatened to prosecute electricians who stopped work over asbestos in August.

“It's pretty clear the system is broken when workers are threatened with legal action for refusing to expose themselves to a deadly substance like asbestos,” Mr McKinley said.