As stated the 2-prong will always work, but why would you???? Unlike the US which is full of dodgy wiring and ungrounded outlets, I've never seen an outlet in Oz that wasn't grounded, even in very old wiring -- so you can always use a 3-prong plug. Given the spectacular effects of 230V failure when not grounded, I'd stick to 3-prong adapters. It is a pretty important safety issue.