Fragments of asbestos have been found in roof panels at the new Perth Children's Hospital after controlled tests, the West Australian Government has confirmed, with exposed workers set to undergo health checks.

Key points: Worker found asbestos in sealed roof panel earlier this week

Worker found asbestos in sealed roof panel earlier this week Contractor John Holland declared hospital asbestos-free in 2013

Contractor John Holland declared hospital asbestos-free in 2013 Up to 25 workers in asbestos-affected area on Monday: CFMEU

It comes after the Construction Forestry Mining and Energy Union (CFMEU) warned hundreds of workers at the $1.2 billion hospital in the suburb of Nedlands may have been exposed to white asbestos.

Health Minister John Day told ABC 720 Perth tests had found asbestos present in four of seven samples taken from a roof panel, supplied by Chinese subcontractor Yuanda to lead contractor John Holland.

"It's so called white asbestos or chrysotile ... it's very disappointing that it is the case, it should not be there," he said.

"It does not accord with what was expected to be provided by the company manufacturing them, and according to the testing and samples that were certified about three years ago as being suitable and appropriate for this use.

"It is not what West Australians expect in the $1.2 billion that is being funded for this wonderful new Perth Children's Hospital."

John Holland said the hospital's roof would need to be replaced, and conceded the work could delay the planned opening of the new facility in October.

There are 150 of the roof panels in question on the eight floor of the building, which John Holland had independently tested in 2013 and found to be asbestos free.

A statement issued by John Holland said that Yuanda Australia had provided them with inspection and test certificates for all its products supplied to the hospital project, including the roof panels, to confirm they did not contain asbestos.

John Holland said action would be taken to "investigate and determine the failure of this process".

The CFMEU said there were up to 25 employees working directly on the asbestos-affected area at the hospital on Monday when one of them cut into a sealed roof panel and made the discovery.

Work continuing despite contamination fears

CFMEU WA secretary Mick Buchan questioned how a banned imported product could end up in a new hospital, and said asbestos could have spread throughout the site.

John Holland project director Lindsay Albonico said 20 workers were in the area where the fragments were left in the open. ( ABC News: Briana Shepherd )

"We took a sample from a working platform ... it was cut through with an angle grinder creating plumes of contaminated dust," Mr Buchan said.

"That dust then could have then filtered all the way down to the ground floor, it could have gone through the air conditioning system ... on the walls, anywhere.

"We've got workers who dusted off their clothes on Monday night, hopped into their car and went home ... it starts a whole train reaction."

John Holland project director Lindsay Albonico said 40 workers had expressed concern over exposure to asbestos and he could not rule out further contamination.

"It's not impossible but we immediately set up airborne monitoring stations in that location and that happened over a 24-hour period," Mr Albonico said.

"There is no evidence of that spreading."

Mr Albonico said 20 workers were in the area over the 24-hours the fragments were left out in the open and the company was providing support.

Matthew Ryan was working when the panel was being cut and said there was a lot of dust. ( ABC News: Emily Piesse )

"We've talked to them about managing their apparel, we've talked to them about what impacts that might have on their family, we've gone to where they would have gone immediately in terms of their crib rooms, we've tested their crib rooms and all of those results have come back negative," he said.

Matthew Ryan, a welder who was working in the atrium area as the panel in question was being cut into, said he could barely see his hands in front of his face at times.

"There was that much dust coming off as they were cutting it," Mr Ryan said.

Mr Ryan said as of this morning, he had still not been offered any support or information through John Holland and he was terrified he may have exposed his family.

"No-one's been in touch with us," he said.

"We've spoken to somebody from John Holland who said yes there were samples found which were positive and that's as far as it's gone.

"I went home and picked up my daughter wearing the same clothes I was working in."

WA Premier blasts John Holland

Premier Colin Barnett described the asbestos scandal as "totally unacceptable", but said he did not believe it would delay the hospital's opening.

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"Hopefully there'll be no risk of health problems for those workers," he said.

"There have been some other issues around John Holland .... I think John Holland will be extremely embarrassed about it. It will affect its ability to win other government contracts.

"So they've got some reputational damage to restore."

But Mr Day said John Holland was taking "full responsibility" for replacing the panels, and said they appeared to have acted in good faith.

He said the company's Australian head Glenn Palin had told him "we've been dudded too".

It was revealed on Tuesday an urgent check had been carried out at the new Royal Adelaide Hospital after the discovery of an asbestos-tainted building product in a Brisbane office tower.

However the checks revealed the suspect gaskets used in Brisbane by Yuanda Australia had not been used on the Adelaide hospital.

It was also revealed last month that SA-based company Australian Portable Camps was being investigated over imports from China which illegally contained the deadly substance.

Asbestos imports to Australia have been banned for more than a decade.