In Australiafirearms laws are controlled by the individual States, laws vary State to State, some states have no controls on owning such magazines, example South Australia, Queensland as I understand. In NSW they would be considered a 'Prohibited Weapon'. Amongst the rare licence holders that still have actual guns, movie armourers, firearms dealers with prohibited weapons licences, odd museum, collectors in certain States, there are numbers of Magazines around for the dollars mentioned above if you have the right licensed contacts. What makes them so expensive is that although the firearms laws are State controlled the Feds control imports and currently there is no real import test where one can import such magazines. Basically what's available in country is it and most were sold to overseas surplus decades ago or destroyed in the 1996 gun buy back. I paid $110 per mag for the last lot I rounded up (I'm a licensed dealer). Same goes for all MG parts out here. Anyone in Australia with L4 barrels or bolts drop me a note, Please!
So Peter, whether you can own a Bren mag in Oz depends on where you live (what State) or what type of Dealers Lic you have. Most common .303 Bren mags encountered out here are MA (Lithgow) manufactured, I'd say 90 plus %. With L4 7.62 Bren mags most are MA but just as likely to be Enfield.
Regards
Daniel