Most Australians will use the Aussie vernacular - in other words, slang. The most well-known is, of course fair dinkum. There's also dinky di, but fair dinkum is the most common. It means it's really true, honest to God. If you ask fair dinkum? that means you're asking if someone's on the level.
Footy is Football (Aussie Rules) - often sounds like foody, just like instead of saying twenty, some people say twenny.
If something's cactus it means it's had it, it's dead. We do have many colourful phrases, which I'll call "Australianisms"
Shot through like a Bondi tram
A stubby short of a six-pack
Not the full shilling
A sandwich short of a picnic
Not all there in the upper storey
The lights are on but nobody's home
Kangaroos in the top paddock
As useful as an ashtray on a motorbike
As useful as tits on a bull
As useful as a pork chop in a synagogue
Flash as a rat with a gold tooth
Dry as a dead dingo's donger
Mad as a cut snake
Bending the elbow
Freeze the brass balls off a monkey
You're like a wet week