Sam.

Check out either:
http://www.sonicelectronix.com/
or
http://www.crutchfield.com/

Sonic do ship direct to Australia but Crutchfield tend to have cheaper prices. The only downside with Crutchfield is that they dont ship direct to Australia so you need to use a mail forwarding service such as shipito to get it here. Shipito charge $8.50 per package plus whatever the cost of postage is to Australia, Crutchfield orders are posted free to Continental US Addresses also. I've dealt with both in the past and have had positive experiences with both. Crutchfield seem to be a little more professional and send through wiring diagrams and installation instructions vehicle specific which is pretty handy if you haven't done it before. Both will also provide a wiring harness adaptor so essentially the head unit will be plug and play.