Prime Minister Alexis Tsipras has called on Greeks to vote 'No' on the referendum, which asks them if they want to accept more austerity in return for bailout funds.
But Germany, France, Italy and European Commission chief Jean-Claude Juncker have warned that this option could be a vote against Greece's place in the euro, and maybe even in the European Union.
"Our whole purpose today is to show Greek people that we're by their side," said Evy Yannas, one of the organisers of a rally in Melbourne which attracted several hundred people.
Australia's southern city of Melbourne is dubbed the third largest Greek city in the world, and many at the rally on the steps of Victorian Parliament held strong connections with the country.
"I just want to let everyone know that Greece is still beautiful despite all their economic challenges at the moment and that they should definitely visit Greece," Greek national Paris Kakridas told AFP.
"A lot of people don't have that option," he told AFP. "They're not Australian citizens and only have a Greek passport... They're doomed living in Greece."
Others in the crowd were sceptical of renewed austerity measures.
"Everyone is innocent," said Christine Dallas. "They want to transfer loans onto the taxpayer of Greece and they come in, they take out assets and the Greeks have got nothing left."
These words were echoed at a much smaller rally in Sydney at which 'No' placards were prominent.
"It's nothing to do with economics, it's purely a political situation," organiser Eleni Pitsilioni-Alexiou told the gathering of several dozen people, adding that Greek people wanted to be "equal citizens of Europe".
