Well, lets break down the design of the patch shall we....
Red text around the border, Aussie flag has the british one which has alot of red, so i thought red should have a place in it, plus, it breaks up the blue well enough. Halo text is there, and Southern Cross Spartans made sense for us Aussies anyway.
Metallic ring is a Halo Ring seen from the side profile.
The 2 Energy Swords, which are set in the "Crossed Swords" design from the Halo 3 multiplayer logos. Done in blue to represent the Australian flags blue
The Master Chief helmet at the back, cause well, he had to be there somewhere.
The Southern Cross, a unique part of the southern hemisphere, a stellar constilation which points due south, used by early navigators as reference. It's on various flags of our region of the world. (notice how they sit on the joins of the crossed energy swords
That is the basic rundown.
But things i DIDN'T WANT, the aussie flag, ODST Marines etc, cause they aren't as well known, assault rifles cause it's southern cross (ed) spartans so the swords suit the title.