If it was a Sussex/Barnevelder cross, I'd expect a darker down color. Australia has Light, Speckled, Red, and Buff varieties. The down seems too pale for Red, or Buff. Speckled chicks have the wild-type chipmunk markings. And it's definitely not a Light Sussex.
Red Sussex is definitely a recognized breed and given the white legs of your chick it may well be a Red Sussex but I've never actually seen a Red Sussex chick (just Speckled and Light Sussex chicks), so I can't say for sure.