Austria's presidential election re-run has been postponed by two months after the glue used on postal vote envelopes was found to be faulty.

Interior Minister Wolfgang Sobotka said the issue - which means postal ballots are coming unstuck - couldn't be fixed in time for the scheduled October 2 vote.

The election has now been set for December 4.

Green Party-backed Alexander Van der Bellen and the anti-immigration Freedom Party's Norbert Hofer have already faced each other once on May 22, when Mr Van der Bellen beat Mr Hofer by less than one percentage point.

This marginal victory largely came down to postal ballots.

In July, Austria's Constitutional Court ruled in favour of a re-run after irregularities were revealed, including the processing of postal ballots sooner than they should have been.

If Mr Hofer wins, he will become the only far-right head of state in the European Union.