More than a year since Perth's $1.2 billion children's hospital was supposed to open, that date is now unknown partly because builder John Holland hasn't found the source of lead that has contaminated the facility's water supply.

The WA government on Tuesday defended awarding the contract to John Holland despite the new hospital being beset by a litany of problems.

Labor has accused the government of lying about the progress of the project, after obtaining internal state government reports through a freedom of information request that reveal thousands of defects at the construction site, indicating a badly managed project.

Those reports showing poor quality control and lax reporting go up until March last year, well before other problems emerged such as asbestos being found in roof panels and lead in the water supply.

“What these documents show is that while the government was telling the WA public that the hospital was coming along nicely (and) it should be finished by 2016, they were receiving absolutely no evidence from the builder that they could back up that claim,” he said.

The state government will seek tens of millions of dollars in liquidated damages from John Holland, which taxpayers have incurred due to the hospital’s delays, but it won’t be able to recoup all of the costs.

It has spent an extra $28 million since July 1 having to keep the Princess Margaret Hospital for Children open longer than was budgeted for, according to its mid-year review.

The government is also spending $500,000 a month to a private contractor the Capella Parking consortium as compensation for lost earnings due to the fact their car park is not being used.

Treasurer Mike Nahan said the government would have been worse off if it had sacked John Holland because it might have lost the ability to make sure they were responsible for the cost of the delays.

Dr Nahan and Health Minister John Day said the hospital would not be opened until the Office of the Executive Director Public Health had given the all clear that water quality was acceptable for human consumption.

Dr Nahan said he believed the acquisition of John Holland in 2015 by Chinese group CCCI had led to staff changes and appeared to have had an impact on the quality of its work.

“John Holland had had an excellent record as a contractor before it took on the contract here, it had just finished the Joondalup expansion (public and private hospital) on budget and on time,” he told reporters.