South Koreans have voted to elect a new parliament, with the ruling conservatives expected to regain a majority despite a sluggish economy and a four-year term considered one of the least productive ever.

Early voter turnout was low compared with four years ago, according to the National Election Commission, a trend if it continues is likely to work to the advantage of President Park Geun-hye's Saenuri Party.

Saenuri holds half of the 292 seats in the National Assembly. Before recent defections, it held a majority with 157 seats in the 300-seat unicameral parliament.

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Voters expressed frustration that parliament has neglected real issues such as jobs and national security in Asia's fourth largest economy and focused more on protecting political interests.

"I hope that parliament will be more mature to mirror the maturity of the voters and that politics can be used for the welfare of children and young people," Kim Jeong-yeon, 46, said after casting her vote in Seoul.

A strong showing by Saenuri would raise expectations that it will field the winning candidate in a December 2017 presidential election to find a successor to Park at the end of her single five-year term.

The country has a strong presidential system around a national leader who is limited to a single term by constitution but has control over domestic and foreign policy issues.

South Korea's economy grew 2.6 per cent last year and youth unemployment reached 12.5 per cent in February, the highest since the government started keeping records in 1999, compared with single-digit joblessness in other age groups.