The Assembly was diverted into Star Wars scenarios during debate yesterday, when Liberal leader Jeremy Hanson dubbed the new office building the ''Death Star II'', a name it acquired when first proposed four years ago.

''Let's stop this nonsense, let's stop the return of the Death Star to the people of the ACT,'' he said, in a diversion that saw Mr Hanson variously compared with Jar Jar Binks and Han Solo, Mr Barr with Darth Vader, and Shane Rattenbury with an ewok.

Mr Hanson accused the government of contemplating ''a sweetheart deal for the Labor mates'', suggesting the CFMEU's superannuation arm, Cbus, was eyeing the project. He challenged the government to rule out interest from Cbus.

Mr Hanson said the office block, omitted from the budget, was part of the government's ''funding arrangement where everything's free'', but whether the government built it outright or paid for it with a long-term lease, the people of the ACT would pay, he said.

The government has called for expressions of interest from the private sector to build the new 42,000 square metre block on the car park beside the Assembly building on London Circuit, or refurbish existing offices within 10 minutes' walk to house 3400 public servants. Registrations of interest close on August 20.

Mr Hanson also baited Greens member Shane Rattenbury over his party's previous opposition to the office block, ''back in the good old days when we had a Green that thought independently'', in light of Mr Rattenbury's support now.

But Mr Rattenbury said the new plan was very different. The Greens' concerns in 2011 about cost, vacancy rates, and lack of consultation with the property sector had been addressed. The government was now looking at a range of options, including retrofitting existing space, he said.

''It is time for many office buildings in Canberra to be brought out of the '60s and into the 21st century,'' Mr Rattenbury said. ''Overall, just as it is with cars, washing machines and fridges, it is actually usually the daily use of energy that makes the biggest difference in terms of overall life-cycle analysis energy use, over that of the embodied energy of the existing concrete.''

Mr Barr said the building was the most economic option available. It would streamline the public service and reduce costs when departments were reorganised. The government was not paying the capital cost, but simply replacing existing leased accommodation with new leased accommodation.

The new block, to house about 3400 of the 20,000-strong bureaucracy, would be ready for late 2018. It would stimulate the economy in a challenging time, and the sale of older buildings would free up the Northbourne corridor for redevelopment.

It also linked to projects such as City to the Lake and the tramline, Mr Barr said.

If built on London Circuit, it will sit near the new convention centre, which will lead over Parkes Way to the lake. One plan for the tram also has it sweeping around City Hill to join the site.

Mr Barr said of the 323,000 square metres of vacant space, only 40 per cent was A grade, much of that at the airport. In the city, of the 77,000 square metres of empty space, only 16,000 was A grade, he said, denying that the market was critically oversupplied.

''There is not [a substantial number of] A grade buildings sitting vacant in Civic or in our town centres that could effectively accommodate ACT public service requirements for around 40,00 square metres in a way that achieves the efficiencies of scale and scope that is being targeted through co-location,'' he said.