Mr Gentleman has called in the $600 million expansion, which was a major election commitment in 2016 when the Labor government pledged it would be open by 2022.

After lengthy delays it is now expected to open in 2024.

Mr Gentleman said he used the powers to prevent further delays, saying it would have "taken a bit longer" to go via the usual route.

The Greens have hit back at the move, with planning spokeswoman Jo Clay saying a need to use call-in powers reflected a lack of consultation or planning.

"The problem with using call in powers is we can get a politicised result," she said.

"It's really important to follow the process we've got set out and make sure that we do all of that consultation and all of those assessments in the right way.

"It's really about planning properly for the project and starting early enough that you can do all of that."

Call in powers allow the Planning Minister to approve major projects that have been subject to consultation, bypassing the normal planning process.

Mr Gentleman said he wanted to ensure the expansion was delivered on time, adding he was confident there had been sufficient consultation and each of the nine submissions made to the development application, submitted in May, were carefully considered.

The approval includes the new emergency department, the closure and reconfiguration of Hospital Road and additional car parks.

It is conditional on the completion of a long-term parking plan for staff and visitors and a revised reconfiguration of Hospital Road North, before construction begins.

"I'm very confident that the planning authority has had a rigorous assessment of that project," Mr Gentleman said.

"It's important for the Canberra community that this project goes ahead in a timely manner ... it's important that a minister can step in and make these decisions to ensure that occurs."

When asked whether the normal process could have played out without delaying the project, Mr Gentleman said that was a "hypothetical question".

"It's something that we haven't looked at in a time sense, how long it might have taken further to achieve that approval through the planning process," he said.

"I've made the decision because I think it's an important decision to ensure that we do keep on track."

Health Minister Rachel Stephen-Smith said there had been consultation on the new emergency department and other aspects of expansion for the past 18 months.

"Consultation that we've done with the community set up the development application to be a really strong application," she said.

Ms Stephen-Smith said consultation would continue while the project was under way.

Consultation on the hospital campus master plan is under way, with options weighing up open space and room to grow.

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"We have spent more than 18 months refining the design of the Critical Services Building to reflect the feedback from our ongoing consultations with hospital users, clinicians, families, carers and local residents.

"More than 250 user group workshops have been held with clinicians while local community and consumer reference groups have been meeting regularly and will continue to do so throughout construction."

Opposition health spokeswoman Giulia Jones said the government resorted to call-in the project after years of setbacks.

"We have continually called on the government to expand Canberra Hospital," she said.

"The failure of this government to do so for over a decade has led to the longest ED wait times in the country and thousands of Canberrans languishing on surgery waiting lists."

Internal demolition of existing buildings in place of what will be the new emergency department has recently begun.

Mr Gentleman also used call-in powers last year to fast-track the Common Ground housing project in Dickson.

In 2019 he called in the new Coles supermarket development in Dickson and in 2018 a major electricity supply project was approved.

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