The Department of Finance, which oversaw the complex's construction, is yet to determine the cause of the broken 2.4-metre by 2.2-metre panel facing Parkes Way.

Monday's broken window is another blow for the $700 million building after a tall panel almost four metres high broke in January.

Almost three years ago another 19 panes, each of which cost $3500, "progressively failed", according to the department, and fell off. These windows had not been fixed permanently to the building at the time.

A spokeswoman for the Finance Department said an engineer would inspect the latest break on Monday, January 5 to determine what happened.

"What caused the glass to break we don't know yet," she said.

"The broken glass panel was located on the south side of the Parkes Way facade. The glass panel will be repaired as soon as possible."

The monolithic office was formally opened in July last year and many of the intelligence agency's 2000 staff were expected to move in two months later.

But the building still remains close to empty, with sources saying only a small team of staff have moved in.