Polls have closed in the eastern states, but voting will continue until 10pm NZT when the booths close in Western Australia.
More than 14 million people were enrolled to vote in the election, with opinion polls indicating one of the closest election results in recent history - or a possible hung parliament.
The coalition needs 17 Labor seats to win the election by garnering a uniform swing of 2.3 per cent across the country. But the government can lose its absolute majority if it drops 13 seats.
Several exit polls showed Labor leading on two-party preferred votes.
The Nine Network exit poll has Labor on 52 per cent of the overall vote to 48 per cent for the opposition.
A Sky News exit poll, conducted by Auspoll in 30 marginal seats, shows Labor leading 51 per cent to the coalition's 49 per cent.
The Ten Network JWS exit polls shows the ALP at 52 per cent to the coalition's 48 per cent, but much tighter in key marginals, while Seven's has Labour 51 per cent over the coalition's 49.
Former Labor prime minister Bob Hawke says he's comforted by exit polls, but he believed a result was some time away.
"I hope we'll be calling a Labor victory, but I'm realistic. I take comfort from the exit polls - I'd rather they were the way they are, both of them showing a slight Labor lead," Mr Hawke told Sky News.
"It is a case where you could have a ... narrow Labor victory, a coalition victory or a hung parliament, and they are the realities.
"And unfortunately we've still got a while before we identify the booths from which the first results are coming in, because you've always got to know on election night, you've got to know your booths - there's a great variation in the booths."
Former Liberal leader John Hewson has predicted the election will fall in Labor's favour or end in a hung parliament.
Dr Hewson, who lost the 1993 election to Paul Keating, said Julia Gillard becoming prime minister marked the start of a comeback for the coalition.
"When Julia took over from Kevin (Rudd), I hadn't given (Opposition Leader) Tony (Abbott) a chance until then, and I thought from then on it was a contest," he told Sky News.
"It's somewhere between a three- or four-seat win to Labor and a hung parliament."
Liberal Senator Eric Abetz praised the efforts of Mr Abbott in getting the coalition within striking distance of victory.
"Win, lose or draw, for Tony Abbott, this has been a bringing of the coalition back from a 28 per cent primary vote to within ... winning," Senator Abetz told Sky.
"I think that is a tremendous record for Tony Abbott, irrespective of the result."
Nationals Senate Leader Barnaby Joyce has predicted a narrow win for Labor, but told AAP Mr Abbott had emerged a winner anyway.
"This is the person who brought the coalition back from a position of being decimated, for decades as some said, to the point where they almost won," he said.
Labor strategist Bruce Hawker has forecast an ALP election victory by a margin of about four to five seats, but has admitted anything could happen as the votes are counted.
But several Labor ministers, including Tony Burke and Stephen Smith, say the election is too close to call and a clear result may not be known before very late.
"I would be amazed if we have a victory speech from anyone by 11 o'clock tonight," Mr Burke told Sky.
