West Australian Premier Mark McGowan says he doesn't believe former treasurer Mike Nahan's claim he was misinformed about lead contamination at the new Perth children's hospital.
A WA Building Commission report released on Monday revealed the likely sources included a now-disconnected "dead leg" section of pipe attached to the ring main for the QEII Medical Centre precinct, which includes Sir Charles Gairdner Hospital.
That contradicts claims by the former Liberal government that the problem was restricted to the new children's hospital site.
Mike Nahan, who as treasurer fielded many questions about the overdue project, said he was told in September that a "pipe buried under the road" could be the source.
But he was not advised it was the dead leg connected to the main water circuit, Dr Nahan told 6PR radio on Wednesday.
"The first time I heard about the dead leg was in the report," the opposition leader said.
Mr McGowan, however, said Dr Nahan would have known.
"I actually don't believe Dr Nahan when he says he didn't know about it," the premier said.
"He was the minister responsible for it for 18 months, there were a range of statements when he and the former premier said that the lead was coming from off site, so I think they have some explaining to do."
Mr McGowan said the project would be probed as part of Labor's commission of inquiry and he also supported a separate parliamentary inquiry.
"I want all the information out there. I don't want any more secrecy," he said.
"All of the information should have come out the entire way along - that's why when we came to office we said we'd release the information and we have."
Dr Nahan said he welcomed the investigation and he looked forward to taking part.
"My commitment always has been, and remains, that we need to open the children's hospital in a timely and safe manner," he said.
"The McGowan government has two tasks ... to assure the public of Western Australia the water in the QEII ring main is safe and second, open the hospital in a timely and safe manner. And I'll do everything to support that."
Mr McGowan said he was confident the lead could be removed using phosphate.
The hospital was due to open about 18 months ago and is costing taxpayers about $6 million per month while it remains idle.