The reason there hasn't been an Aussie World Champion is related to Epstien Barrs syndrome.
A similar disease - but extremely contagious - infected 97% of the Aussie pro motocrossers circa 1975. Rahier Barrs Syndrome is thought to have been accidentally imported via the Bert Flood dealership. The first local to go down with it was Victoria's Mike Landman.
Before long it had spread interstate. Symptoms included drooping elbows and a lethargy that left riders circulating mainly on their seats. Butt-punished and whoop whipped, they never quite got up to ultimate speed. 1983 Mr MX R VDB is one notable exception who, despite being afflicted, flopped and flapped his way to this country's ultimate victory that year.
Meanwhile, the Belgians stood up ... and took most of the global titles. Notably, they were short. Wee guys like Eric Geboers didn't seem to suffer too much from Rahier Barrs as, being short-legged, they had trouble getting their arses off the seats anyway