Somebody from Oz please to pipe up. I need to you proof-read something for me, and edit it so that it's "in Australian" (as opposed to some guy from Florida trying to write a letter like an Australian).
I did that for a novel I wrote about ten years ago. I had an Aussie friend through Usenet, a reservist in the army. Sent the scenes by e-mail, let him mark them up. Not just slang and idiom, but cultural norms in the Australian Army. I had a book of Aussie slang, but it took a native to get me things like "Corporal Meyer. Not a question, a comment, hey? I grew up a bit north of your op area there. Pop gave it a burl as a cocky, but he never made a crop. No water. The bush there is uglier than a hat full of armpits, pretty rough for a Yank galah."
I have no idea if that works for a native (maybe my helper was pulling my leg . . .), but US readers liked it. Did the same thing for a skydiving scene, some Brit-speak, prison slang, etc.
No-one really speaks like that ... you might use *one* of the phrases, but some of them are pretty obscure. "Burl", "Cocky", "Bush" and "Galah" are the most likely ...
Of course, in an extended conversation you might see all of them used, but, seriously, even then its unlikely ... Australian Standard English is really not all that colourful in actual use ... certainly in the big cities and, unusually, Australia has always been demographically dominated by the big city populations, especially Sydney and Melbourne.
You're much likelier to get things like the diminutive "-zzies" or "-ssies" ... so "prezzies" for "presents" or "cossies" for "swimming costumes" ("budgie smuggler" is known, but, really, almost never used except as a sort of joke) ... and always pronounce "Aussie" as "Ozzie" rather than "Ossie" ... nothing annoys us more
Apart from that, the sorts of things that are really different are mainstream idiom rather than slang ... so, for example ...
Footpath not Sidewalk
Ground Floor not First Floor
Chips rather than French Fries
Biscuits rather than Cookies
Toilet rather than Bathroom
Hung rather than Hanged
Boot rather than Trunk (car)
Bonnet rather than Hood (car)
... and more. Note: Some of these are in the process of change, mostly due to the influence of US TV shows on local television, but the differences were accurate for WW2.
There are some regional idioms as well, but, again, relatively few ... the only one I can recall offhand is that in NSW and Queensland, and, I think, Victoria, what you may call "Lunch meat" in the US is "Devon", yet in South Australia (lotsa German settlement there in the Barossa in the late 19th century) it was "Fritz" ...
Unlike the US and UK (as I understand it) there is almost no regional variations in pronunciation ... though there are a few ... for example, in Sydney we call the town to the north (Newcastle) "Newcarssle" while in Queensland it was often (and may still be, haven't been there for a while) "Newcassle" ... unless you're Prof. Enry Iggins II you would find it impossible to tell where an Aussie is from, specifically, by their pronunciation.
Author, Space Opera (FGU); RBB #1 (FASA); Road to Armageddon; Farm, Forge and Steam; Orbis Mundi; Displaced (PGD)