The contractor of the delayed Perth Children's Hospital claims the facility could have opened months ago, but the premier says the company is trying to dodge its responsibility.
John Holland's region manager Lindsay Albonico told a parliamentary inquiry on Friday the lead contamination was sufficiently resolved by April and would not have prevented the hospital from opening.
Mr Albonico also disagreed with the WA government that brass fittings within the hospital were the source of the lead problem, saying rather it was a build-up of material in the water ring main.
But Premier Mark McGowan said the state government had to ensure the highest standards of testing of the water to protect children and staff.
"I think John Holland has not behaved well in relation to this hospital and I support high, stringent standards," he told reporters.
"John Holland is a multinational company that mostly likely will be sued over the issues with the Children's Hospital, so naturally they're trying to get themselves out of responsibility.
"I would take the advice of the chief health officer whose role is to protect the health of children and staff."