The removal of asbestos sheeting from within roof panels at the new $1.2 billion Perth Children's Hospital is "a patch-up job" that shows how poorly managed the project has been, the opposition says.
The asbestos was discovered almost six weeks ago and was blamed as a major reason the lead contractor, building company John Holland, missed the recent handover date.
Removing the asbestos-riddled fibre cement boards from inside their galvanised iron shell - rather than completely replacing the panels - has been trialled, with consulting company Coffey concluding hazardous materials had been satisfactorily removed.
But the CFMEU is concerned about the plan, saying it is too risky and the panels should be completely replaced.
"It seems to be built down to a price and not up to a standard," a union spokesman told AAP.
Opposition health spokesman Roger Cook agreed, accusing John Holland of cutting corners.
"John Holland are trying to do the cheapest possible job they can because they have to pay to replace the panels," Mr Cook told reporters.
"We know the government are desperate to get this already delayed hospital open as a matter of urgency, but we are calling on the government today to do the job right.
"Don't try and do a patch-up job, stop trying to repair the panels, replace the panels.
"We want to make sure that future generations of sick kids who will be accommodated in the building have got a proper facility."
The union also says it has been repeatedly denied right of entry to the building site, excluding one occasion.
Health Minister John Day, who hopes the hospital can open its doors to patients by December, has repeatedly expressed disappointment in John Holland, which was originally expected to complete the building by the end of last year.
Since then, it has emerged the company installed 935 fire doors that may not comply with Australian fire standards and 450 metres of faulty water pipes, all of which it had to replace at its own cost.
Mr Day warned the company after it missed the handover date earlier this month that it may be pursued for liquidated damages.
Separately, Chinese building materials company Yuanda, which provided the panels, is being investigated by Border Force for suspected illegal imports.
The Nedlands facility will replace the ageing Princess Margaret Hospital for Children.
