The State Opposition says an independent inquiry into the Peel Health Campus must go ahead, despite revelations the key witness is wanted in the United States for alleged fraud.
Former campus employee Ashton Foley made serious allegations against the hospital's operator, Health Solutions, to a Parliamentary committee last year.
She is due to give evidence to a new Government inquiry this month.
Last night, Ms Foley admitted she is wanted in the US for alleged fraud and identity theft.
"What is clear from the Parliamentary committee's report, is there is substantial documentary evidence, there is substantial testimony from other expert witnesses and there are substantial allegations which need to now be addressed," Mr Cook said.
"So regardless of what people might think of the veracity of her claims, the fact is that hospital has substantial allegations which need investigating."
"It's important the Government undertake those investigations so the public can have confidence in that hospital."
The Premier Colin Barnett declined to comment.