The vacated 35,000 square metre Princess Margaret Hospital (PMH) site in one of Perth's prime real-estate locations is likely worth more than $100 million, the Urban Development Institute Australia has said, as uncertainty surrounds the future of the site.

PMH closed its doors yesterday after more than 109 years caring for WA's sickest children, with the final 91 patients transferred to Perth Children's Hospital (PCH) as it opened its emergency department.

The PMH Subiaco site, which is bounded by Roberts Road, Thomas Street and Hay Street, will be remediated and mothballed before eventually being handed to the Department of Lands (DOL).

But the State Government has already said the site has "heritage issues" and that it was unsure what its future held.

Urban Development Institute Australia (UDIA) WA president Nick Allingame said land of that size so close to the Perth CBD, Subiaco and West Perth was a "once in a generation" opportunity for developers.

"It's smack bang between West Perth and Subiaco," he said.

"You've got West Perth, which is evolving from an area of offices to a more residential area, [and] you've got all the amenity of Subiaco on your doorstep and the new school … so it's a fantastic opportunity."

Mr Allingame said while he thought developers would have "a lot of interest" in the land, there were other factors at play.

PMH has already been fenced off ahead of decommissioning and remedial work. ( ABC News: Eliza Lashon )

"Ultimately it's going to be up to how the Government wants to use the site, because they may wish to use it for public purposes," he said.

"It's a prime piece of real estate. It's really going to be dependent on how much is usable once they have sorted out the remediation and the heritage issues.

"I would imagine there is a substantial part of the site that actually can't be utilised."

The current heritage-listed PMH buildings are Godfrey House, on the corner of Roberts Road and Thomas Street, and the former outpatients building — most recently a multi-faith centre — on the corner of Hay Street and Thomas Street.

No instant windfall, Government warns

Lands Minister Rita Saffioti said the site would undergo about 14 weeks of decommissioning work before it became the responsibility of the DOL.

Ambulances transferred the last 91 patients from PMH to Perth Children's Hospital yesterday. ( ABC News: Emily Piesse )

"We're preparing a business case for the PMH site, together with the surrounds, including the area around Subiaco Oval," she said.

"Sometimes with these sites in particular — with the number of sites you're dealing with, trying to demolish parts and keep parts — the costs do add up."

Treasurer Ben Wyatt also warned the site would not bring an instant windfall for the Government, but was confident it would be valuable in the long run.

"We won't be rushing it to market, and I suspect we probably can't rush it to market, because a fair bit of work would have to take place," he said.

"I suspect that there will have to be some money spent on it to bring it to a point where it's ready for sale.

"[But] I think the location makes it a particular attractive sale in the event and when Government goes about selling it, so I'm fairly relaxed about getting a good return on it."

'A legacy for future generations'

Mr Allingame said it would be important for any developer to understand and acknowledge the cultural significance of the PMH site.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 45 seconds 45 s A look back at Princess Margaret Hospital

"PMH holds a near and dear place in every parent's heart and for all of us as West Australians, because we've seen the great work that has been done there," he said.

"That's going to be a very important part of whatever happens with the site, acknowledging that history and being respectful toward it and making sure it is a legacy for future generations — that will be a major consideration for any redevelopment that the Government will lead.

"I think if they rushed it that would be controversial … once it is redeveloped, the site is then effectively quarantined for another 50 to 100 years."

PCH dispute resolution 'a way off': Wyatt

Meanwhile, Mr Wyatt said negations with head contractor John Holland over the Perth Children's Hospital construction project were not yet resolved and could still end up in court.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 14 seconds 1 m 14 s The saga of Perth Children's Hospital

An inquiry last year was told the builder would be liable to pay the state at least $42.5 million due to delays to the hospital, while John Holland is seeking $300 million in compensation from the Government for changes to the project.

"The legal issues and ongoing negations will continue, that's a way off being resolved yet, but at the moment there's still no official legal proceedings going," the Treasurer said.

"Once it ends up in court it becomes very protracted and long and expensive, so I hope not, but we'll work that through."