Since 2008 an estimated three million passengers a year have passed through the terminal re-construction site unaware of the cutting edge infrastructure being installed under their feet.

In a warren of halls and rooms below and besides the public areas a gas-fired tri-generation power station, long banks of switch boards and 22 kilometres of cabling layered on trays keep the place heated, cooled and operational as a high security facility.

A back-up diesel plant with enough capacity to run 90 per cent of the airport for four days is in place should the power station, or main grid fail.

As well, a bank of batteries sits on standby, able to power essential functions such as security for hours on end if the diesel option is lost.

The western concourse will mirror the southern concourse terminal which was completed in 2010 and occupied by Qantas. An eye-popping 28m-high atrium will separate the two concourses.

Virgin Airlines remains in the old terminal, which is being dismantled in stages, and will move into the western concourse once it's completed.

The new terminal will have capacity for more carriers, international travellers and associated services such as Customs.

Construction Control senior project manager Ross Cleaver said staging of aircraft parking, passenger and vehicle access and parking, taxi services, rental cars and keeping all of the services and utilities going while replacing everything in the precinct had been one of the main challenges.

"It's like a game of chess, getting access to these areas," Mr Cleaver said while walking across new sections of the airport apron. "And I'm not good at chess," he added.

More than 80,000 square metres of new apron is replacing 30-year-old, heavily patched bitumen. Parked planes put the heaviest stress on the apron, and stand on 400mm thick concrete with heavier aggregate to handle the load.

Crane working heights are limited at various points around the precinct and aren't allowed to operate in fog as Canberra's biggest privately-funded infrastructure project inches forward.

Mr Cleaver said all the heavy lifting for the atrium, including the installation of 17 tonne steel beams and nine-metre long glass panels, had been completed.

Apron works, including the demolition of the remaining old terminal should be finished in November next year.