WA's health minister has defended not speaking to doctors before moving the location of the state's new $1.8 billion maternity hospital 20 kilometres away from its current location, saying no amount of consultation could have changed the government's mind.

It comes as documents presented to state parliament show that decision was made just weeks after the government received an initial report on the proposal.

Then-Premier Mark McGowan shocked doctors and the community in April when he announced the new women's and babies' hospital would be built within the Fiona Stanley precinct in Perth's south, abandoning plans to integrate it with the centrally located Sir Charles Gairdner and Perth Children's Hospitals (PCH).

Doctors and the opposition have criticised the government for failing to consult on the change and highlighted the potential risks of moving the hospital further from specialised services at PCH.

But Health Minister Amber-Jade Sanderson has maintained the disruption caused by the major building works needed at the Queen Elizabeth II (QEII) Medical Centre would be too risky and drag out the new hospital's completion from 2029 to 2034.

Business case released

Today, after months of questions, the government tabled in parliament the business case which it says backs up those claims.

That report was handed to government in March 2023, the same month it received a "desktop" analysis into whether the new hospital could instead be built at Fiona Stanley.

A review of the government's decision by one of its own agencies, Infrastructure Western Australia, said cabinet considered both reports in April and publicly announced the move shortly after.

That review supported the government's plans, and agreed the risks of building at QEII were too great.

Doctors caught on the hop

Criticisms from doctors have been led by the Australian Medical Association, which had questioned why the government did not speak to doctors about its decision before abandoning the QEII plans.

Health Minister Amber-Jade Sanderson today acknowledged the government had known about the risks, including construction impacting medical services at Sir Charles Gairdner, as the business case was being developed.

But she defended making the decision to change location within government, and then consulting doctors after it was announced.

"And the fact is, these risks cannot be reasonably mitigated.

"I realise that this came as a surprise to people. I think the vast majority of the community want us to get on with building our women's and newborns' hospital.

"That's what this decision will allow us to do."

Ms Sanderson said the need to replace the ageing King Edward Memorial Hospital quickly also contributed to the decision to move sites.

$228 million blowout

The 181-page business case, which the government says sank plans to build the hospital at QEII, estimates it would have taken 10 years to build and come in $228 million over budget.

That document was reviewed by the government's infrastructure agency four months after it was handed to the Health Department.

Infrastructure WA said it agreed with government's decision that the risks of building at QEII were big enough that alternative locations needed to be considered, and pointed out a further decade of disruption could follow once construction was finished.

"It is evident the FSH precinct is a less constrained … site that presents a more straight forward delivery option," it concluded.

But it stopped short of recommending Fiona Stanley as the best alternative saying further work, which is underway, was needed to understand the risks of that site.

Ms Sanderson maintained the maternity hospital needed to be located alongside one of the state's tertiary hospitals – either Royal Perth, Sir Charles Gairdner or Fiona Stanley – with the latter the only viable option.

"In a perfect world, the new maternity hospital would be built alongside Perth Children's Hospital and we did genuinely try to achieve that," she said.

"However, we also did the appropriate due diligence that the people of Western Australia expect us to do on a $1.8 billion project.

"We could not have ignore what that due diligence found. It would have been completely irresponsible to do that."