Hey Rob Garmin or Magellan, is probably the most used Gps for that type of thing.
Havn't kept upto date with what's avalable now, GPSOZ are here in monavale and you will get expert advise there,to suit your needs, rather than your local dicksmith store
http://www.gpsoz.com.au/Johny appleseed is another gps specialist shop .one in nth Sydney, though am not sure on what advice you would get there
http://www.ja-gps.com.au/Gps Australia is a good forum with plenty of info.plus they sell gps as well
http://www.gpsaustralia.net/