The airport's Brindabella Business Park is making a determined pitch to provide the 80,000 square metres of office space needed by the merged department, and real estate insiders believe the site, in the city's east, has put together a strong case to land the massive tenancy.

But there is stiff competition with other developers around town, and Immigration's present landlords are not prepared to walk away from the potentially huge deal.

The new department is determined to have all of its Canberra-based public servants under one roof and within 10 kilometres of Parliament House. And Belconnen businesses are increasingly worried that Immigration's 4000 public servants are heading out of the town centre.

With few big deals on the horizon for Canberra's struggling commercial property market, several developers are chasing Immigration's signature, and the competition and political lobbying are gathering intensity.

Canberra Airport Ltd declined to discuss the matter with The Canberra Times, but real estate sources say the industry understands Brindabella Business Park is one of the few locations in Canberra where 80,000 square metres can be provided under one roof.

The newly merged, 13,500-strong department has specified it wants its Canberra-based public servants all in one building or no more than 400 metres apart.

The airport precinct already has public service tenants: the Defence Department, micro-agency the Professional Services Review and the Australian Federal Police.

The area's retail credentials have also taken off. It now hosts Australia's largest Woolworths supermarket and US discount retail giant Costco. And Swedish furniture behemoth Ikea is close to opening a giant new shop nearby.

Immigration's current landlord, Amalgamated Property Group, is understood to be fighting hard to hang onto its prized tenant and staying put in Belconnen could prove the cheapest option for the new department.

But that plan would see Customs "bolted" onto their new colleagues, probably in a new or refurbished building near Immigration's current headquarters.

Elsewhere, the joint venture between property giants Leighton and Mirvac is keen to attract the giant new government tenant to their Section 63 site on London Circuit in Civic.

The companies paid a record $92 million in 2008 for the car park, and have been waiting for a tenant ever since.

Another landlord understood to be chasing Immigration's signature is the Walker Corporation, which needs a deal with a big occupier to kick-start its $300-million dream of a massive redevelopment of a large slice of Civic.

Walkers want to build three 11-storey office buildings with shops and restaurants on a site bordered by Northbourne Avenue, and Cooyong, Bunda and Mort Streets, but the project is going nowhere without a large office tenant to anchor it.

The Immigration Department refused to discuss its real estate quest.

Acting assistant director Phil Allan said: "We have nothing further to add on this matter - updates will be made available in due course."

A self-imposed deadline for Immigration to make a decision on its future accommodation passed in March without a choice being made.