'Two countries separated by a common language' applies just as well to Antipodeans as it does to Americans. It seems they insist on abbreviating everything to the shortest, most casual form possible.

The classic examples (gleaned from years of Neighbours watching) are:

Afternoon → Arvo
Chicken → Chook

But how about...

Breakfast → Brekkie
Strawberries → Strawbs
Pedestrians → Peds
McDonalds → Maccers
Vegetarian → Vego
A Politician → A Pollie!

I'm not even sure if these can be called slang either, as they're used by absolutely everyone, regardless of how formal the occasion. Having been in Australia for the build up to the recent general election, we tuned in to watch the first of the televised debates when we were in the Blue Mountains. The leader of the opposition Tony Abbott began his opening statement, 'I want to give the Australian people a fair go.' A Fair Go!!! This is apparently his key campaign message! He then went on to use the phrase 'Fair dinkum' twice in the first five minutes before we turned the TV off.

A variation on pure abbreviation, there's also a fair amount of very literal slang in Australia. My favourite is the name they give fruit machines over here: 'Pokies.' I was buying some beer in a 'bottle-shop' and asked the shop keeper if they were screw caps or not: 'Twisties? Yeah I reckon.'

P.S. One last bit of slang worth mentioning purely for its crudity. Apologies, but - Shit-arse. As in 'Are you working this weekend?' 'Yeah... Shit-arse.'