To legally drive in Australia you need to have your car fitted with official number plates.
The characters on the number plate has to be visible from at least 20 metres away and the number plate has to be permanently attached to the car at all times. Number plates are helpful in ways like helping to identify your vehicle in the case of an accident or car theft.
Of course it is your responsibility to keep your number plate in good condition and if you do not replace damaged number plates or have an illegal number plate you can be fined up to $320 AU dollars.
source: http://www.quebella.net/autos/australian-number-plates/

Jon Cherry is a Director of leading personalised number plate dealer Regplates.com. Jon has over 25 years industry experience handling some of the most expensive plates ever sold with many high profile and celebrity clients. Active since 1991 in the number plate industry, Jon is currently Chairman of the Cherished Numbers Guild, a trade body representing number plate dealers in the UK. Jon has written many articles on the industry and insight into the future of numberplates and the market as a whole.