Between 16 and 17 million people are expected to vote across the vast island-continent, with the centre-left Labor party tipped for victory after six years in opposition.
Casting his ballot in Melbourne, would-be prime minister Bill Shorten was confident of forming a majority government after a final poll showed his lead increasing.
"Today is the people's day," he said. "In the event that the people of Australia vote to stop the chaos and vote for action on climate change, we will be ready to hit the ground from tomorrow."
Weeks ago, Prime Minister Scott Morrison's conservative Liberals had been heading for an electoral drubbing.
But he has closed the gap with backing from the country's biggest media organisation owned by Rupert Murdoch, mainly targeting older, wealthier voters who face fewer tax breaks under Labor.
After casting his vote, Morrison acknowledged the challenge his coalition faces, saying, "I don't take anyone's support for granted."
Anger over his government's inaction on climate change may prove the real difference between the two parties. A season of record floods, wildfires and droughts have brought the issue from the political fringes to front and centre of the campaign.