G'day Andrew, you need to ask a few questions first, like what purpose do you want it for, car or hand held? Obviously for bush tracks, but do you want it for use on the black stuff as well?
If you are wanting an in car gps for bush tracks I would suggest something that would run OziExplorer or the like so then you can add your own detailed maps of certain area's.
Have a read through the gps section of the forum and see what others are using. Also have a look at this site
GPS Australia .:GPS Australia:. if you really want to be left scratching your head.

I believe now there is a 'dongle' that can be plugged into your laptop which can then be used as gps, but don't know much about it.
Never an easy choice, after some research you will find what you are after. I run a cheap chineese gps with Igo8 and Ozi loaded on it, a good mapping garmin handheld and with the great tools we have now like google earth etc and of course the trusty paper maps and compass I will always find my way there and back.
Not much help, but something for you to have a think about.
Cheers