Offroad Maps Australia (oztopo, track4australia, oztopo contours, shonkymaps, opensource) 4x4 Australia

Just load them on to a microsd card and slot them straight in to your gps. Go in to settings then maps then click info and select which map you want.

I personally prefer to use Oztopo as it has the most clarity and I can even switch it to contour mode which shows me the altitudes of the hills and mountains.