The collapse of a ceiling section at Perth's Children's Hospital has been blamed by the construction union on building firm John Holland doing a "rush job" on work to remove asbestos from inside roof panels

The company says it believes the Gyprock plasterboard was damaged in heavy rain on Sunday night but is yet to identify how the water got in, causing a section to collapse.

Exclusion zones are in place while work is underway to identify any further damage.

A John Holland spokeswoman says the affected section is next to atrium roof panels containing asbestos sound-proofing sheeting, but it’s not yet clear whether the collapse is related to work to remove the hazardous material.

Asbestos was discovered within the panels, supplied by Chinese company Yuanda, by a worker nine weeks ago and a decision was made last month to cut into them and remove the sheeting.

The CFMEU said at the time that the panels should be replaced and reiterated that call on Wednesday.

State secretary Mick Buchan said John Holland had been removing several panels at once – not one at a time as agreed – and the rain entered the ceiling cavity because plastic sheeting used as temporary waterproofing was inadequately secured.

“We had a concern from the start when we went through the methodology that to remove the asbestos in situ will create a vast range of hazards and concerns, and increases the risk of the problems that have occurred,” Mr Buchan told AAP.

“We had that extreme weather on Sunday evening – from what I gather five panels were removed and some temporary waterproofing was put on that leaked, which then filled the ceiling cavity underneath.

“It’s just a rush job. You can’t fix these problems by taking shortcuts.”

The John Holland spokeswoman said the remediation plan did not stipulate replacing the atrium panels one at a time.