Austria's presidential election was too close to call on Sunday, meaning postal ballots were set to determine whether an anti-immigration candidate would become the European Union's first far-right head of state.

A victory for Freedom Party candidate Norbert Hofer would be a landmark triumph for parties across Europe that have capitalised on Europe's migration crisis and widespread dissatisfaction with traditional parties of power.

It would be all the more remarkable for being in a prosperous country with low unemployment, where two centrist parties have dominated since it emerged shattered from World War II after its annexation by Nazi Germany in 1938.

"The sovereign has spoken," Hofer's opponent, former Greens leader Alexander Van der Bellen, told broadcaster ORF. "What exactly it has said - Hofer or Van der Bellen - we will know tomorrow afternoon."

A projection by the SORA institute for broadcaster ORF, based on 100 percent of votes cast in polling stations and an estimate of the outcome of postal voting, showed a statistical dead heat on 50 percent each. The margin of error was 0.7 percentage point.

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The provisional result from the Interior Ministry, which did not include postal ballots, showed Hofer ahead with 51.9 percent to van der Bellen's 48.1 percent.

Postal votes will not be counted until Monday and their exact number is not known. They tend to be used by the more highly educated, a spokesman for SORA said, a group among which 72-year-old Van der Bellen has greater support.

Interior Minister Wolfgang Sobotka said he expected there would be about 750,000 postal ballots, roughly 12 percent of Austria's 6.4 million eligible voters.