The ATO's newish office was supposed to co-locate all the Head Office staff, which is pretty much all the Canberra staff.

It was designed to house 4,200 people.

There are a lot of departments with many more than that number of employees in Canberra.

Defence (not even counting the armed forces)

Foreign Affairs

Immigration

FaHCSIA

Human Services

Probably a few others - Health, Education...