Expressions of interests are expected soon for the ACT's contentious $432 million government office building in the city, while the Commonwealth is expected to call for expressions of interest within six to eight weeks for new offices.

Colliers International ACT chief executive Paul Powderly said Mirvac and Leighton were refining their plans on the Section 63 car park on London Circuit to bid for both expressions of interest.

Leighton and Mirvac paid a record $92 million in 2008 for the land and have been waiting for a tenant ever since.

Mr Powderly said the 2.6-hectare site was one of only three big enough to meet the requirements of the expressions of interest.

The other sites were developer Lang Walker's Myuna complex between Northbourne Avenue and Mort Street and QIC's car park in Cooyong Street behind the Canberra Centre shopping mall.

The ACT government also had land that could accommodate its new office block.

Mr Powderly said the Commonwealth was looking for office space to combine several offices where leases would expire at about the same time, for a transport and infrastructure agency.

He said that in the lead-up to this year's federal election no one was spending money, and talks were about what would happen next year and the outcome of the election.

That outcome would also determine the name and structure of the new transport agency.

Major developments such as the new ACT office block will shape the CBD and complete the fine-grain detail of two major infrastructure plans released in March.

''The timing of the City Plan and City to Lake Plan are a fair way out and one would hope that Leighton has started their development pretty well before then,'' Mr Powderly said.

''These sites are going to have to be developed for the City to Lake and City Plan to really get some meat on the bone.''