Native Fish Australia
http://www.nativefish.asn.au/fish.html#exotics
Fish files include information on Australian freshwater species that are native and exotic.
South Australian Native Fish Association
http://www.sanfa.org.au/
This web site includes a copy of the Freshwater Fishes of the South Australian Gulf Division poster - click on the pictures for a description of each fish species.
Australian Museum
http://www.austmus.gov.au/fishes/
Find-a-fish fact sheet including some freshwater species, identification keys, FAQs, fish videos (marine fish only), information about the fish collection and research, and student stuff for secondary and tertiary students including fish dissections.
Streamwatch NSW - Gambusia fact sheet
http://www.streamwatch.org.au/streamwatch/resources/file/ebdedf435947f08/gambusia.pdf
This site includes everything you need to know about this little introduced pest as well as colour photographs of the males and females, and natives that it can be confused with.