Conservative challenger Tony Abbott was heading for a major victory over Labor Prime Minister Kevin Rudd on Saturday as millions of Australians voted in national elections.

Polling booths opened at 8:00 am (2200 GMT Friday) on a warm spring day with 14.7 million electors taking part in a mandatory ballot across the huge country where early exit polls showed Abbott on track for a landslide win.

A Sky News exit poll released before voting was to end at 6:00 pm (0800 GMT) predicted that his Liberal/Nationals would gain a massive 25 seats to sweep 97 of the 150 seats in the lower House of Representatives.

The survey, carried out by Newspoll, forecast Labor would lose 21 to be left with just 51. The independents would have two seats.

On a two-party basis, Abbott's coalition would take 53 percent of the vote to Labour's 47 percent.

A separate Morgan-Channel Ten exit poll showed that in the primary vote, which takes into account the minor parties and independents, the conservatives had 42.5 percent to Labor's 33.5 percent.

The Greens Party would garner 11 percent and the newly-established Palmer United Party, run by colourful billionaire Clive Palmer, five percent, with "others" taking the rest.

Rudd has struggled for traction after toppling Julia Gillard, Australia's first female prime minister, as Labor leader just weeks before calling the election and his party seems destined for a period in opposition.

He may not be around to lead it with the Sky poll showing he could be toppled in his Queensland seat of Griffith by Liberal/National Bill Glasson, with the result too close to call.

"It's 50-50," Newspoll chief Martin O'Shannessey said, reporting a seven percentage point swing away from Rudd.

The prime minister nevertheless remained upbeat ahead of casting his ballot in a Brisbane church where he was met by a group of noisy refugee advocates who yelled at him about Labor's mandatory detention of asylum-seekers who arrive by boat.

"It is days like this that we celebrate a democracy, people get to chose their government," Rudd said.

"I believe we have put our best foot forward. I'm very confident in people's judgement because they will assess what is best for our country's future, their community's future and their family's future."

Asked if he would step down if he lost, Rudd said: "This is politics. You take things one step at a time."

A relaxed Abbott, 55, running as opposition leader in his second election, said he was ready to assume the leadership.

"Inevitably, all candidates are nervous but I am confident I am ready and my team is ready," he told reporters at the Freshwater surf club in Sydney, where he cast his ballot with wife Margie and three adult daughters.

Abbott has made a paid parental leave scheme his "signature" policy, while pledging to scrap the carbon tax and make billions of dollars of savings to bring debt down.

Rudd, also 55, has campaigned on his administration's success in keeping Australia out of a recession during the global financial crisis.

He has also promised to scrap the carbon tax brought in by Labor after the 2010 election and move to a carbon emissions trading scheme by July 2014.

Other key policies include a plan to introduce a bill in parliament to legalise gay marriage and the adoption of tough measures to halt asylum-seeker boats.

Voters turned out in force early and many admitted they had tuned out of the campaign and made up their minds some time ago, with the toppling of Gillard by Rudd and instability at the top of Labor a common theme.

"I would have voted for Julia Gillard," one woman said while voting at the Bondi Beach public school, referring to Australia's first female leader. "Labor lost me forever with that."

Most polls close at 6:00 pm Sydney time and the remainder two hours later on the West coast, with the elections deciding the make-up of the lower house and half the 76-seat Senate.

Despite the logistical difficulties in such a large country, Australians overwhelmingly abide by their obligation to vote, turnout never falling below 90 percent since it became compulsory in 1924.