Asbestos has been found in roof panels used to construct the new Perth Children's Hospital, the state government has confirmed.

The 15-centimetre thick panels are fully encapsulated in metal and contain cement boards.

A number of workers have signed a asbestos register following the revelation at Perth Children's Hospital. Credit:AFR

It was found the cement boards, located on level 8, could contain asbestos after a worker cutting a hole in the roof to install a smoke exhaust fan noticed an unusual white substance on Tuesday night.

An initial test indicated the presence of asbestos but the sample was potentially contaminated after being placed in a worker's toolbox, Health Minister John Day's office confirmed.