There are (3) official holidays in Dec 15-Jan 15: Dec 25, Dec 28, Jan 1
All other days would be people taking annual leave. As this is also school holidays many Australians take Summer holidays during this period and many businesses either shut or slow down during this period.
There is a note on DIAC website regarding all DIAC offices and call center closure dates. The note says they are closed from December 24th to December 29th. The note also says they will be working on December 30 and December 31.