In my research it is suggested that there have been only 2 plebiscites in 1916 and 1917. Where does the national anthem question and same-sex marriage question fit?
Ballot box graphic
Parliamentary Education Office (peo.gov.au)
This graphic is a representation of a ballots being placed into a ballot boxes.
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Yes, there have only been two plebiscites. A plebiscite is used to decide a national question that does not affect the Australian Constitution. The Australian Parliament decides how plebiscites are run. The national anthem question was a public opinion poll and the same-sex marriage question was a postal survey. Both of these asked Australians what they thought about an issue but weren’t called plebiscites.