i seriously doubt australian breweries keep 4-5 millions kegs in stock. That would mean with just lion nathan and cub there would almost be a keg for every 2 people in australia.
Fosters' keg float is worth on the order of $400,000,000 and they lose a chunk of it every year to all sorts of stuff. Mainly theft for scrap metal.
To give you an idea of the volumes... At the Abbotsford brewery, if they start the day with a few thousand empty kegs in the yard... and no one brings back any empties... by early afternoon production stops to wait for some to come in
In a truly spectacular case of "borrowing" one year about 100 kegs were found being used to keep a jetty afloat on the Murray river. The decking of the jetty consisted of "borrowed" chep pallets and there were a number of milk crates attached that went into seats and benches etc.
The deal is basically this... if its got a brewery's name stamped, etched or painted on it... its not yours, not even if you bought it from the scrap dealer or the tip, or your mate.... Unless you have a letter on official letter head, from the brewery that owns the brand name, saying that they sold it to you and that you are the legal owner. If you are buying a branded keg from someone, if they haven't got such a letter, then they have no right to sell you the kegs.
If you are talking "old style" 18 gallon kegs or casks or the sort of things that just aren't used anymore... then you're probably fine... but if its a modern Sanke style keg... then someone wants it back.
I'm not saying anything bad about those of you who choose not to send them home, dont really give a rats, but just so you know.