Watchout for connection ("flag fall") charges on Australian SIMs. Also when you see a value posted like $500 worth of calls. Think of the monetary value as credits or units. What they are saying is that you can use or units for text, voice, or data. Many plans however use artificially high calling rates and connection charges to get to their $500. I've seen domestic mobile calling valued at $0.89 a minute in the worst example.
I like Amaysim and put my brother and his family on it. A great domestic resource on prepaid values is:
http://forums.whirlpool.net.au/forum/114