Holocaust denial per se is not an offence in Australia.is holocaust denying considered against the law?!
Just as it not an offence to deny Armenian genocide, nor the Rwandan genocide,
nor the work of Pol Pot ("The Killing Fields"), nor the genocide of Bosnian Muslims
during the break-up of Yugoslavia.
That said, depending on the circumstances
(that is, what somebody says, the where, and the how),
such a statement could amount to unlawful racial vilification.
Indeed, as JohnR points out above, it already has.