Revelations that high levels of lead have been found in the water at Perth's new children's hospital have further embarrassed building giant John Holland and the WA government seven weeks after asbestos was found on the site.

John Holland is yet to find out the cause of the toxic material, but in a statement said metallurgical testing had ruled out taps and valves it is responsible for.

However Master Plumbers Association of WA chief executive Murray Thomas said it was highly unlikely that mains water that the Water Corporation quality tested and put out would have lead in it before it got to the hospital site.

“I am not pointing fingers at the moment … but I would suggest it’s possibly not the water supply, I’m just thinking about the products in the water supply system,” he said.

“It’s just unbelievable, lead just doesn’t happen in water supplies in Australia or the world at the moment, it’s mindboggling.”

He said the products at the site needed to be inspected to make sure they were WaterMark compliant with plumbing standards, especially if Chinese-made, as the roof panels found to contain asbestos were.

WA had just eight plumbing inspectors compared to more than 30 in the past, Mr Thomas said.

John Holland said it was using a water quality consultant to identify the cause.

WA Plumbers union secretary Brian Bintley said workers had known about the lead contamination since last month but were ordered not to talk about it.

Treasurer Mike Nahan said the children’s hospital was currently a construction site, not a hospital and he was confident the problem would be fixed.

However Dr Nahan has been one of John Holland’s fiercest critics as problems plague the hospital, including delays in its opening of at least a year, widespread problems with subcontractors not being paid and multiple products having to be replaced at a high cost.

The government will pursue John Holland for damages payments over the delays.

Opposition health spokesman Roger Cook said it was unacceptable for asbestos and lead to be anywhere near a children’s hospital and the government was mismanaging public infrastructure projects.