I just spent a week vacationing in Australia and enjoyed the experience thoroughly. I understood what everyone said about 95% of the time.

One Aussie phrase that I'd never heard before was "road duplication." I had no idea what that meant but learned that it means "widening the road." (We were passing a road construction crew at the time.)
If you say "put another shrimp on the barbie," Aussies will just roll their eyes because they NEVER say that; it got started back in the '80s by Paul Hogan as part of an American advertising campaign for Australian travel. Australians say "prawns." Apparently Hogan changed the prhase to "shrimp" because it would be more palatable and understandable to Americans.
Another Aussie phrase is "Pizza with the lot." That means "a pizza with everything on it." (Yes, it comes with prawns, too.)
By the way did you know that Vegemite has an EXPIRATION DATE?