The delivery of the expansion - known as SPIRE - will be managed by the newly created agency Major Projects Canberra on behalf of the two health departments.

As recently as December, the government said construction on its 2016 election promise was due to start in 2020.

But tender documents reveal construction is now expected to begin in 2021.

The agency is looking for external legal representation to help it go to market and find a construction contractor.

The successful legal firm will act for the ACT in contract negotiations for the delivery of SPIRE.

The project was the government's major infrastructure announcement before the 2016 election, when it said it would open in 2022.

Chief Minister Andrew Barr has denied the government will go into next year's election without starting construction on the project.

The expected completion date has since blown out to 2023/24.

The plans have been slightly expanded since the initial plan was submitted to treasury in 2016 for costing.

Earlier this year the government announced a further 12 intensive care beds - bringing the total number to 60 - and two more operating theatres than previously announced.

When open it would have 114 emergency department beds, 22 operating theatres and 64 inpatient beds.

There are four fewer emergency department beds in the current scope than what was initially pledged, according to the latest tender documents.

The recently released government tender seeking external legal representation said an early contractor involvement procurement method would be used for the project.

This means the successful contractor would be involved in the early stages of design and would not just come into the project during construction.

This decision was made due to the complexity of the project and to ensure construction could start in early 2021.

A construction company would not be appointed to build SPIRE until the middle of next year, with the chosen legal firm to be involved in the selection and negotiation process.

The contract for the successful legal firm to assist in the procurement will be awarded in August.

"The Territory is seeking to engage a law firm with specialist construction experience to provide legal advice and assist in the formulation of the contractual framework to inform the approach to the market and subsequent engagement of a construction contractor for SPIRE," tender documents say.

The tender said Major Projects Canberra will release expressions of interest to the market to build SPIRE in late 2019.

The initial contract will be awarded in mid 2020 while construction would begin in the first quarter of 2021.

While there has been speculation the project will not be finished until 2025, the government still plans on finishing the project in 2023/24.

The government's plans for SPIRE had to change course last year, after its initial pick for the location of SPIRE on the Canberra Hospital campus was ditched due to logistical challenges and cost blowout fears.

Health Minister Rachel Stephen-Smith said construction would begin once design and site preparations were finished.

"The initial phases will include decanting, demolition and other early construction works on the site, this work will overlap with the design of the project," she said.

"Preparatory site works for SPIRE will get underway with decanting and site works commencing this year, with construction set for completion in 2023-24.

"The project remains on track and the previous guidance provided by the ACT Government remains accurate. Delivery of the project is now proceeding.

"The procurement by Major Projects Canberra of a legal advisor is just one example of how the ACT Government is moving forward with the delivery of SPIRE."

Mr Barr rejected the suggestion construction was not going to start before next year's election.

"Yes, construction would have started and we committed in relation to that project to get construction underway," he said.