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.

Perth Children's Hospital developer John Holland in need to explain latest toxin scandal.

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.