South Korea's left-leaning ruling Democratic party was heading towards a parliamentary majority in Wednesday's general election, exit polls showed, as voters backed President Moon Jae-in's handling of the coronavirus epidemic.

A survey by national broadcaster KBS projected the Democratic Party and a sister organisation would win between 155 and 178 seats in the 300-member National Assembly, while MBC and SBS also predicted it would secure an absolute majority.

The main opposition United Front Party and its sister grouping were expected to take between 107 and 130 seats, according to KBS.

The country uses a mix of first-past-the-post and proportional representation, and while the Democratic party was the largest in the outgoing parliament it did not hold a majority, relying on minority support to pass legislation.

Turnout was at least 63.8 percent, the highest since 2000.

Moon's position was not at stake as he is directly elected, but just a few months ago he was being assailed by critics over sluggish economic growth and his dovish approach to nuclear-armed North Korea.

But analysts say South Korea's relatively quick and effective handling of the coronavirus epidemic was  a boon for the incumbent government ahead of the vote, largely seen as a referendum on its performance. 

"With the virus spreading to other countries and South Korea receiving a relatively good evaluation in comparison, the epidemic has become an advantage," said Hahn Kyu-sup, a communications professor at Seoul National University.