Prolly a mutt. The body type tends to dominate in crosses, but their sizes vary a great deal when another breed is involved. My guy, for instance, has a heeler body (stocky, short legs, high round but, stumpy tail), but he's got a German Shepherd coat and weighs between 60 and 70 pounds, depending on the time of year and quality of his diet.
Here's more info on 'em.
http://www.dogbreedinfo.com/australiancattledog.htm They're a very recent breed, which probably contributes to the wide variation in morphology. They've got some dingo mixed in, too, which I'm convinced is where my boy gets his weird high-pitched vocalizations. (Generally very quiet workers, cattle dogs are. Good cattle rustling dogs.)