From our experience, the Australia Theme can crop up on three types of occasion.
Firstly, on Australia Day (January 26th) or when Aussie teams are involved in sporting activity.

 

 Secondly, when a relation or work colleague is emigrating or taking a long holiday 'down-under'. Aussie corked hats are usually part of the requirements here

Thirdly, in mid-summer, some genius decides it is a good idea to declare an Aussie Christmas Party. Good opportunity to swelter in a Santa suit or perspire in a pixie outfit. Mid-summer also offers the opportunity for a costume mash-up cross between Barbie and Lara Croft - a Barbi-girl!

 

The Australia theme offers a small range of costumes