AFAIK no GPSs sold outside Australia are loaded with Australian maps.
If you buy a 900 series TomTom Go here in Australia, it will have Australia + NZ, as well as the major world maps (USA, Canada, Europe) also loaded. So if you don't already have a GPS, maybe consider buying one when you arrive.
The alternative is to buy a TomTom or Garmin which has the facility to upload additional maps, which you can buy online. The TomTom Go 6XX, 7XX and 9XX are certainly up to scratch for this purpose.