AC/DC isn't metal. And really? Dead Kelly? Borry? They're not even the best Aussie bands from their respective subgenres, much less the authors of Australia's greatest metal songs. If you wanna talk the best, go to Buffalo, Sadistik Exekution, Bestial Warlust, Destroyer 666, Hobbs' Angel of Death, Armoured Angel, Blood Duster, Pegazus, Mortal Sin, etc.
By who? The same brain trust that christens Led Zeppelin a metal band? AC/DC are hard rock and blues rock, proto-metal at most. They lack several of metal's definitive musical traits, including the infusion of classicism and jazz stylings among other traits pioneered by Sabbath.
Never heard anyone saying they considered ACDC metal before§
Anyway, there are lots of cool australian metal bands, in about any metal subgenre one can think about, so pickin ONE best band seems quite hard to me!