Perth's new Women and Babies Hospital will open on time, but its $1.8 billion budget may blow out, the state government admits.

Construction on the new maternity hospital in the Fiona Stanley precinct officially began today, under plans to replace the more than 100-year-old King Edward Memorial Hospital (KEMH) in Subiaco.

Health Infrastructure Minister John Carey said the 12-story, 274 hospital was on schedule.

"We are on schedule, we are very clear that we are committed to delivering and opening this hospital in 2029 and anyone who doubts that can just look at the site right now," he said on Tuesday morning.

"Earthworks are underway and ... piling works will begin in November."

Mr Carey said while the $1.8 billion price tag for the new hospital was accurate, he could not rule out an

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