This building will cost more than $1 billion. What do we know about it?

A stone's throw from Parliament House, six giant cranes have been working on the biggest Commonwealth construction project in Canberra in almost 40 years.

The development of a National Security Office Precinct has been heralded by the federal government as a solution to "critical accommodation and capability requirements" of several agencies.

Despite the huge scale of the project, many of the details have been kept secret.

Here's what we know.

Who will use the precinct?

We know the precinct in Barton will be utilised by a range of Commonwealth entities.

That includes the Office of National Intelligence and the Department of Foreign Affairs and Trade (DFAT).

In Senate estimates in May, we heard some details that there may be more to come.

"We are engaging with other agencies as part of the development for the precinct going forward," Nick McClintock of the Department of Finance, which is leading the development, said.

Next to the main buildings, there will also be hospitality and retail venues, servicing both the wider public and workers in the precinct.

Up to 5,000 workers are expected to be based at the precinct.

Why is it needed?

At the sod turning for the project, Finance Minister and ACT Senator Katy Gallagher said a "purpose built" and "specifically designed" building was needed to allow the national intelligence community "to do the work they need to do in a changing world".

"[It's] clearly needed in terms of our national security agencies being able to work in a facility that actually allows them to do their job properly."

She said the precinct would be "more appropriate" and "more secure" than existing buildings, with several government agencies reaching, or at, capacity.

How much will it cost?

The project's price tag has not been revealed because of "commercial-in-confidence and national security sensitivities".

But, we know it will cost at least $1 billion.

"It is the most significant investment we've seen in the territory in a long time," Senator Gallagher said in 2023.

She also told Senate estimates: "it is a very significant infrastructure spend" and has also described it as a "multi-year billion-dollar investment".

The final cost is expected to be revealed publicly, but only after construction is completed.

The precinct is the biggest Commonwealth construction project in Canberra since the development of the new Parliament House, which opened in 1988.

Senator Gallagher has previously rejected claims Canberra had missed out on its fair share in recent federal budgets, pointing to the precinct as an example of spending in the territory.

When will it be ready?

Construction is expected to be completed by the end of 2028, with the precinct starting to be occupied from 2029.

The federal government has said the project will support more than 10,000 jobs over the construction period, which commenced in late 2024.

Due to the "security requirements" of the project, it was exempt from needing the green light of the public works committee, which scrutinises major public capital expenditure projects.

We also haven't seen any artist impressions or plans for the precinct, which means we won't know exactly what it will look like for a while yet.

The precinct is being built on the site of a former car park.

What happens to the buildings after agencies move out?

A Department of Finance spokesperson said as existing buildings were "privately owned commercial properties", their future use is a commercial decision for the landlords.

We know DFAT is consolidating its presence in the ACT into two new buildings — the precinct and another new building also being built in Barton.

It means its current head office, the RG Casey building, will be vacated.

The lease is due to run out in 2031, and as noted by the department, its future after that will be up to the private landlord.

It was revealed in Senate estimates in June that staff will be moved into the new precinct as soon as they are "reasonably able to".