Why do we have federal elections every three years?
Ballot box graphic

Parliamentary Education Office (peo.gov.au)
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This graphic is a representation of a ballots being placed into a ballot boxes.
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Great question!
In Australia federal elections are held at least every 3 years, or earlier if requested by the Prime Minister and approved by the Governor-General.
Section 28 of the Australian Consitution says that each House of Representatives convened after a federal election will continue for 3 years from the date of the first meeting but not any longer.