[Tony Smith]

Yes, most of these trucks do ply the Hume Highway on their way to Melbourne, but they also travel to other places in Australia, too.
These are road-going semi trailer trucks, perhaps what you have in mind are the Northern Territory road trains carrying cattle with 3, 4,or even 5 trailers in tow! Any travellers coming to Australia in 2005 for Back to the Track will see one of these behemoths as the trip will end up in the Alice Springs Transport Hall of Fame.