Immigration bosses want to spend more than a quarter of a billion dollars revamping their Canberra headquarters in what is thought to be the most expensive office fit-out in Australian government history.

While the supercharged Department of Immigration and Border Protection - the result of a 2015 merger between immigration and customs - insists much of the $255 million cost of its "Headquarters Project" will be rolled into the lease costs, the politicians asked to approve the plan have raised major concerns.

Under the proposal, the number of buildings the department inhabits in the national capital would shrink from 12 to five, with the main office located inside a new state-of-the-art structure next to Canberra Airport.

The fit-out would include "a unified watch floor, situation rooms, briefing rooms, incident rooms, operation planning rooms, associated intelligence support rooms, and surge rooms". The new building would include an armoury, conference and training facilities, evidence rooms and map rooms. The department envisions an "innovative" and "modern" new office with "large efficient floor plates to support future flexibility" and an open environment that will "promote collaboration and positive cultural renewal".