Asbestos has been found at the site of a hospital redevelopment in NSW, the CFMEU says.

Workers' lives are being put at risk at an active NSW construction site where asbestos was found, the CFMEU says.

The union says it was notified late on Friday that asbestos had been found on the site of the $600 million redevelopment of Sutherland Hospital in southern Sydney suburb of Caringbah.

Union organiser Nicholas Rekes said the union officials arrived at the site on Saturday morning to find workers were still operating near an area where asbestos was visible.

Asbestos was also found in the soil near where work was continuing, he said.

Mr Rekes said he asked builder Richard Crookes to at least move workers further away from the possibly contaminated area.

"This shows a complete contempt for workers' safety on a government job that is ironically about delivering better health care services," Mr Rekes said in a statement.

He said NSW Health Minister Jillian Skinner needed to step in and stop work at the site until it had been cleared of asbestos.

A Health Infrastructure spokeswoman said suspected asbestos material had been returned from the tip on Thursday, but no traces of asbestos were found in the material when it was tested on Friday.

She said although inspections on Saturday afternoon uncovered no asbestos, a piece of suspected material had been roped off.

Testing would be carried out first thing on Monday morning, she said.