The building was originally scheduled to be completed by October. The estimated date of practical completion is now December 18.

An ACT government spokesperson said the government was in discussions with Capital Property Group about the reason for those delays.

"Under the Agreement for Lease, the reason for delays must be supported by detailed explanation and evidence and assessed against the provisions of the Agreement for Lease," the spokesperson said.

The ACT government has also not made any decision on whether to waive its right to claim liquidated damages as a result of the building's delays, the spokesperson said.

Construction stalled on the site last year amid an external review of the project's design. The review, completed by engineering firm WSP, led to design changes which have not been disclosed.

The ACT government said at the time it had been made aware of "allegations" of the review and it had launched its own investigation.

The tender for the project was awarded to the Snow family company in July 2016 and construction began in December 2017.

A five-storey office building will house about 1700 ACT public servants, while a 12-storey building will house commercial offices and a five-star hotel.

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Capital Property Group's head of property Richard Snow said a delay of a few months on the multi-year project was a good outcome. He said the minimal delay through the COVID-19 pandemic meant the project was mostly on track.

"The ACT government building at Constitution Place is due to be practically complete by the end of 2020, following which the ACT Government will move into the building early in the new year, this has been a time frame we have been working towards collectively for a significant period of time," Mr Snow said.

The ACT government has signed a new long-term lease at the Nara Centre to accommodate Civic-based staff who will not relocate to the new Constitution Place offices.