Under the original policies, .org.au was effectively "miscellaneous", it was expressly for domains that don't fit under other categories. .asn.au was for formally registered associations (e.g. not-for-profits). Similarly, .net.au was just for network operators.
When auDA was created in the late 90's, the policies were harmonized and .com.au and .net.au got the same policies, .org.au and .asn.au got the same policies, etc. It has further evolved since based on policy panels I think they conduct every 5 years.
Wikipedia says it was Robert Elz at U of Melbourne. Which squares with that department spinning out Melbourne IT, who had a quasi-monopoly on .au registrations for a while.
My favourite is, of course, .id.au.