Sydney, May 18 (AFP) Polls opened Saturday in Australia's nail-biter election, a race that may be the first anywhere decided on climate policy.

Around 17 million people are expected to cast their ballots across the vast island-continent, as final surveys predicted a centre-left Labor victory.

Prime Minister Scott Morrison's conservative Liberals had closed a yawning gap on the opposition upstarts, but differences over climate may prove the difference.

A season of record floods, wildfires and droughts has brought the issue front and centre in the campaign.

Labor has pledged ambitious targets for renewable energy, while the Liberals said they would not risk the coal-fuelled economy's health to make the air cleaner. Final polls show the vote is going down to the wire, with Labor ahead roughly 51-49.

But compulsory voting and a complex system of ranking candidates mean an upset is possible.

There are growing signs the Liberals' stance on climate may be politically unsustainable.

In rural areas, climate-hit farmers are demanding action, while eco-minded centre-right independents are running them close in once-safe suburban seats that have become make or break for the six-year-old government.

"This will be the closest election we've seen in many, many years," Morrison predicted while making a final pitch to voters in north Queensland.

Weeks ago, the contest looked like it might be a rout for Labor. But a final survey by Ipsos Friday showed Morrison's coalition trailing Shorten's Labor 49 to 51 percent, from 48 to 52 percent two weeks ago.

In some battleground seats, the race is even tighter, with the electorate split 50-50.

"I don't think anyone... thought this is where the election would be the day before," Morrison said.

The campaign has been an often ignominious pitched-battle, with Morrison -- in lock step with Rupert Murdoch's fiercely conservative media -- mounting a relentlessly negative campaign, warning a Labor government will wreck the already slowing economy.

Out on the campaign trail, candidates have been egged, abused and a slew have resigned for racist, sexist and otherwise jaw-dropping social media posts.

In one Sydney battleground seat, a 62-year-old man was arrested and charged with thrusting a corkscrew into the stomach of a man putting up campaign banners on the eve of the election.

Morrison is scraping for his political life, hoping to avoid entering the history books as one of the shortest-serving prime ministers in Australian history.

He took office last August after a party room coup that ousted moderate pro-climate leader Malcolm Turnbull -- the latest in a series of political fratricides that have made Canberra politics look like "Game of Thrones" meets "The Hunger Games." Much of Morrison's cabinet has resigned or gone into virtual hiding because of their unpopularity.

If he wins, it would be one of the greatest political comebacks anywhere, akin to US president Harry Truman's defeat of Thomas Dewey in 1948.

If Shorten is elected, he would become the sixth prime minister sworn into office in a decade.

The former union leader has struggled with low personal approval ratings but has become a more polished campaigner as the election has neared.

Still, his relative lack of charisma was underlined Thursday by the death of much-loved former prime minister Bob Hawke, an Oxford-educated lovable rogue, equally at home chugging a pint or debating Keynesian economics.

But the upswelling of sadness about Hawke's death could remind voters of less contentious times under Labor.

Shorten's hopes of grabbing the top job may hinge on results in Queensland and his home state of Victoria -- where Labor's lead has proved more resilient and where climate change has been a critical issue.

Should he win, Australia will likely get a vote on becoming a republic and, as Shorten put it, returning a head of state that Australia has borrowed from the other side of the world for more than two centuries.

Polls opened at 8:00 am local (2200 GMT) and the first exit polls are expected around 10 hours later. (AFP)

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