Thanks for your question.
A plebiscite can be used to help decide a national question that does not affect the Australian Constitution. When Australia has held plebiscites in the past, it has not been compulsory to vote. However, there are no specific rules for how a plebiscite should be conducted. If another plebiscite is held, it is likely that the federal Parliament would decide on how it would be conducted, including whether it was compulsory to vote.
For more information, check out this fact sheet.