Meanwhile, a right-wing, anti-immigrant party made gains.
With more than 60 per cent of the vote counted, the Socialist Party had 26.5 per cent backing and its centrist People's Party partner was at 23.7 per cent. That was a loss of more than 2 per centage points each for both parties.
A final official tally was expected later in the day.
The right-wing Freedom Party had 22 per cent, a gain of nearly 4 per centage points, after a campaign strong on anti-immigrant and European Union-skeptic sentiment.
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Also clearing the 4-per cent hurdle needed to get into parliament were the liberal NEOS at 4.6 per cent and the populist Team Stronach of Austro-Canadian billionaire Frank Stronach with 5.9 per cent.
The partial results reflected some erosion of support for the mainstream Socialists and People's Party and their pro-EU stance, particularly in the context of the gains by the Freedom Party and Team Stronach's relatively strong showing in its first federal election.
Many Austrians are opposed to their government's financial and moral backing for the bailouts of Greece and other economically struggling eurozone countries during the protracted crisis that has gripped some of the 17 nations using the EU's common currency.