The preliminary result of Austria's presidential vote on Sunday showed far-right candidate Norbert Hofer ahead of independent Alexander Van der Bellen, but by a margin so narrow that as-yet uncounted postal ballots were set to prove decisive, according to Interior Ministry data.

With 4.48 million direct votes cast, the nearly 900,000 absentee ballots issued will likely make them the likely vote decider.

Hofer was in the lead on 51.9 per cent of counted votes, ahead of former Greens leader Van der Bellen on 48.1 per cent, the data showed. Both candidates said the result would not be known until Monday, when a count of postal ballots was due to finish.

The SORA polling institute said the race was a statistical dead heat in a projection for state broadcaster ORF.

Supporters of right-wing Freedom Party presidential candidate Norbert Hofer attend his final election campaign rally in Vienna on Friday. (Joe Klamar/AFP/Getty Images)

If Hofer's slight lead is firmed up, it would put a right-wing politician into the office for the first time since the Second World War.

A win for Hofer, seen as against immigration, would be viewed elsewhere in Europe as evidence of a further advance of populist Eurosceptic parties.

The two candidates of the governing coalition of the Social Democratic and People's parties were eliminated in last month's first round.

A vote for Hofer would be seen as reflecting deep disillusionment with the political status quo and the coalition's approach to the migrant crisis and other issues.

As voting got underway Sunday, the contest was too close to call between Hofer of the right-wing Eurosceptic Freedom Party and Van der Bellen of the Greens Party, who is running as an independent.

Van der Bellen campaigned in favour of 'an open, Europe-friendly, Europe-conscious Austria.' (Joe Klamar/AFP/Getty Images)

Both men drew clear lines between themselves and their rival as they went into Sunday's race.

At his final rally Friday, Van der Bellen said he was for "an open, Europe-friendly, Europe-conscious Austria."

Asked as he arrived to vote Sunday what differentiated him from Hofer, Van der Bellen said: "I think I'm pro-European and there are some doubts as far as Mr. Hofer is concerned."

'This is not your home'

Hofer, in turn, used his last pre-election gathering to deliver a message with anti-Muslim overtones.

"To those in Austria who go to war for the Islamic State or rape women — I say to those people: 'This is not your home,"' he told a cheering crowd.

Later, Hofer sought to soothe international fears that he is too far to the right on the political spectrum.

The Austria Press Agency cited him as telling foreign reporters Sunday that he is "really OK," and "not a dangerous person."