VRT forecast heavy losses for the Christian Democrats and the liberals, former partners in government.
The French-speaking Socialists, whose leader Elio Di Rupo has been widely tipped to become prime minister, were expected to gain six seats to give them 26 overall.
N-VA has called for splitting Flanders and the poorer French-speaking region of Wallonia into separate entities.
Such a split would greatly affect Wallonia, which depends largely on Flemish funds.
No national parties
Voting is obligatory in the country of 10.5 million people where no political party operates nationally.
The electoral system - effectively two elections with separate parties seeking votes from the majority of Flemish and those of French-speaking voters - means at least four parties will be needed to form a governing coalition.
"We did a study of 10,000 people and found 84 per cent want the country reformed, but not broken apart."
Marianne Thyssen, Christian Democrat
Sunday's elections were called one year early, after the five-party coalition of Yves Leterme, the prime minister, fell apart on April 26 in a dispute over a bilingual voting district.
That spat has pushed the N-VA into the top position in the polls.
However some in Belgium say they favour more self-rule for the country's different language regions, but no division.
"We did a study of 10,000 people and found 84 per cent want the country reformed, but not broken apart,'' the Associated Press news agency quoted Marianne Thyssen, a Dutch-speaking Christian Democrat, as saying.
Strict language rules determine which language is used in Belgium on everything from mortgages and traffic signs to election ballots and divorce papers.
In 2003, the country's constitutional court ruled that the bilingual voting district comprising the capital, Brussels, and 35 Flemish towns bordering it, was illegal because it violated the separation of Dutch- and French-language regions.