I love the word G'Day, but you won't hear it that much in Australia.
I know that may sound weird given what you have heard or read about Australia. The funny thing is that you WILL hear it, just not on its own.
A familiar greeting in Australia is: G'Day mate how you going? This is said quite fast so that it becomes one word such as Gdaymatehowyagoin.
It is not intended as a real question but more of a greeting. A real answer to the question such as 'good mate' or 'i'm fine' will work just the same as if you say 'hi' or 'gday' back.
If you're in Australia give it a go!