Sorry to hear about this. Looks like I should be asking for a receipt when do my refills should I rent next time I go to Australia.

Hertz in Korea (well it's not really a Hertz, but a partner) actually has a separate digital display that shows how much fuel was used and what the the current tank level is); I found that kind of neat seeing I knew exactly how much I started out with and that was recorded on my rental agreement. They actually gave me a refund for (1) returning earlier than originally scheduled, (2) returning with more gas than I started with.