A section of ceiling has collapsed at Perth's new children's hospital on Tuesday in the latest drama to befall the site, whose opening is nearly a year overdue.

No one was injured in the incident, which an employee at the site told WAtoday had happened around 2pm.

"Workers were working on the ground floor when the ceiling fell down from the ninth floor," he said.

"Everything is on the floor... and they closed that side of the building."