As an Australian author who writes Australian characters in Australian settings, it's likely I'll use terms unique to our country Down Under that might confuse readers from around the world. Feel free to drop me a line letting me know of any you come across that aren't listed below and I will include them :)
Ambo - short for ambulance, what we call a paramedic (paramedic is becoming more frequently used nowadays)
Buckley's and none - a term used to explain a zero chance of something happening (William Buckley escaped from a Tasmanian prison and was said to have zero chance of survival in the wilderness - he did, thanks to the local Indigenous he befriended)
Esky – a brand of ice-cooler; we use it to describe all ice-coolers (not just those made by Esky)
Larrikan - someone who likes to joke around; a lovable prankster
Mobile phone - what Americans call a cell phone; sometimes referred to as just a 'mobile'
Paddock basher – an old, beat-up car (often a ute - see below); often used in rural towns or for running errands on farms – always look worse-for-wear; like they've gone through hell (see Urban Dictionary here)
SUV - probably not that different to US style SUV's - a family suburban utility vehicle, a smaller 4WD-looking vehicle (sometimes is AWD)
Toorak Tractor - a large SUV/4WD often driven (unnecessarily) by mum's from the ritzy Melbourne suburb of Toorak - where there is simply no need for a 4WD/AWD type vehicle, but driven with the intent of showing power/class/wealth
Ute (pronounced YOOT) – what we call an American pick-up truck, though ours are usually smaller – like a flatbed attached to the back of a sedan. More recently, Australian utes have grown in size and now look like pick-ups (not to be confused with an SUV.)
Whirly whirly - a mini-tornado-like whirlwind; often carrying dust. Sometimes called a dust-devil.