Elections by law happen on a Saturday. That means that they are held in places like school halls, churches, town halls, local community centres and what not.
Barbecues are put on by local Parents and Citizens' (P&C) associations, churches, other community groups etc., as a way of fundraising.
Voting is compulsory in Australia (we have had +90% for more than a century) and because everyone has to vote, it is a good way for P&Cs to raise money.
A real-time crowd-sourced map of sausage and cake availability at Australian elections. It's practically part of the Australian Constitution. Or something. #demsausage
democracysausage.org
Democracy Sausage is so much a part of Australia now, that even Twitter and Facebook have a Sausage icon next to hashtags for elections.
This picture is of the Prime Minister Scott Morrison, at the 2019 Australian federal election, manning a barbeque on Election Day.