Same sex marriage is still unavailable in Australia but is available in a number of other countries. Whilst same sex partners are recognised as de facto couples in Australia under the Family Law Act 1975, same sex marriages from other countries are not recognised as valid marriages in Australia.
The Marriage Act 1961 defines marriage to be "The union of a man and woman to the exclusion of all others voluntarily entered into for life". Therefore, despite significant pushes in recent times to pass legislation to change the Marriage Act to allow same sex marriages, it remains impossible in Australia and also goes so far as to not recognise same sex marriages from overseas.
It has also been deemed not possible for a State of Australia to pass legislation allowing same sex marriages as it will be inconsistent with federal law. Tasmania tried this with the Same Sex Marriages Bill in 2012, but it was not passed for that reason.
Accordingly given the growing push for legalising same sex marriages in Australia, it is necessary for this to be done at the federal level.