"Is there any legal reason why we could not hold the plebiscite on gay marriage at the same time as the election?"

Great question! Generally speaking, no there is not.

The Australian Electoral Commission (AEC) is the authority in charge of administering plebiscites in Australia. According to the AEC, a plebiscite is an issue put to a vote which does not affect the Australian Constitution. A plebiscite is not defined in the Constitution, the Electoral Act or the Referendum Act. No specific rules exist about the running of a plebiscite.

For more information about plebiscites, check out the AEC's webpage What are referendums and plebiscites? The PEO also has a factsheet detailing plebiscites and referendums.

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