The long-running lead saga at Perth Children's Hospital has finally been resolved, clearing the way for the $1.2 billion facility to open in May.

Key points: Water tests show hospital lead levels are below the maximum allowance

Water tests show hospital lead levels are below the maximum allowance Health chiefs say there has been a 'dramatic improvement' in test results

Health chiefs say there has been a 'dramatic improvement' in test results A public open day will be held at the hospital in April after patients move in

The State Government has confirmed WA's chief health officer has given the all-clear for the hospital to open, following nearly three years of delays which have cost the state more than $115 million.

That includes more than $20 million in compensation payments for the operators of the empty hospital car park.

The decision to approve the facility's opening followed the results of water testing, showing the levels of lead to be below the maximum allowance.

That included hundreds of samples being taken at various locations in the hospital.

The first patients will move into the hospital in May as part of a gradual transition from Princess Margaret Hospital.

"It is all systems go," Health Minister Roger Cook said.

"The water at PCH is safe to drink."

Taxpayers have forked out about $20,000 for bottled water at the site during the lead saga.

Roger Cook says it is "all systems go" at the hospital. ( ABC News: Jacob Kagi )

The lead contamination in drinking water was one of several problems that have plagued the hospital's construction.

A bipartisan parliamentary committee report released last week found criticism of the performance of head contractor John Holland was warranted, following issues including long-running lead contamination in water and the installation of asbestos-tainted roof panels.

'Dramatic improvement' in test results

Nearly 98 per cent of the samples taken in the latest round of testing were below the lead level guidelines, above the required 95 per cent level.

Patients will start filling up wards at the hospital in May. ( ABC News: Andrew O'Connor )

That is an improvement from last June, when just 74 per cent of tests produced acceptable results.

"This indicates a dramatic improvement and a remediation of the risk," chief health officer Tarun Weeramanthri said.

"I am happy to give the green light to turning on the water supply at Perth Children's Hospital.

"The [Health] Department cleared the bar with some space to spare."

Resolution of 'wicked problem' welcomed

The department's director-general David Russell-Weisz said the issue had been unprecedented and complex.

"We knew this was going to be a wicked problem," he said.

"We are a 'go' for opening in mid May and mid June."

The hospital was originally due to open in August 2015 before problems set in. ( ABC News: Andrew O'Connor )

The hospital had originally been due to open in August 2015.

Australian Medical Association WA president Omar Khorshid said he had confidence the hospital's water problems were a thing of the past.

"The public can be absolutely guaranteed that there are safe levels of lead in water at the children's hospital," he said.

"We can look forward to a world-class, new facility."

Opposition Leader Mike Nahan, who oversaw the project for several years as the former government's treasurer, said the hospital would serve WA for "generations".

Legal battle looms over costs

The resolution to the lead problem follows the Government's decision to replace brass components of thermostatic mixing valve assembly boxes, within the hospital's water supply system.

Leaching from those brass fittings was identified by authorities as the primary cause of the lead contamination, with Dr Weeramanthri saying the dramatically improved results vindicated that assessment.

Chief health officer Tarun Weeramanthri says the test results speak for themselves. ( ABC News: Jacob )

"The results speak for themselves," he said.

John Holland has long disputed the fittings were the cause of the problem, repeatedly claiming the issue was coming from outside the hospital.

Fears remain that a lengthy legal battle will now be needed to resolve the dispute between the Government and the builder.

The Government believes it is entitled for substantial compensation from John Holland for the delays.

"We will be seeking a very large amount of that from the managing contractor since we do hold that issue," Dr Russell-Weisz said.

But John Holland has firmly rejected that position, previously arguing the hospital was ready to open long ago.

Public open day planned for April

Approximately 300 staff are already stationed permanently on the PCH site and have been drinking from bottled water since the lead issue emerged.

The delays at the hospital in Nedlands have cost the state tens of millions of dollars. ( ABC News: Andrew O'Connor )

The Government also announced a public open day would be held at the hospital in April.

Mr Cook said he believed the facility could become the best children's hospital in Australia, and its quality would quickly allay any public concerns.

"Once people start to see the hospital in operation and feel the hospital in operation, they will be very confident in the services they get from it," he said.