The prediction by the state election watchdog comes a day before people cast their votes to pick the country's next chief executive and is based on a nationwide poll it commissioned earlier in the month.
The survey of 1,500 people by local pollster Korea Research Center showed 79.9 percent claiming they will definitely vote.
NEC said that the percentage of people who said they would vote is similar to the 80.5 percent tallied ahead of the 2002 presidential race. In that race between liberal candidate Roh Moo-hyun and conservative opponent Lee Hoi-chang, the final turnout reached 70.8 percent.
"Judging from past surveys, and actual turnouts there seems to be about a 10 percentage point gap between what people said and how they acted," said an election official.
The official said because the race may be close, the victor of the election may be decided after 11 p.m. although the time is subject to change if a clear lead is maintained from the outset of the vote counting by either ruling Saenuri Party presidential hopeful Park Geun-hye or her main opposition party rival Moon Jae-in.
In the 2002 election won by Roh, it was called a little past 10:00 p.m. with the president-elect holding a press conference at 10:30 p.m.
The NEC, meanwhile, said that it may not be until 2 p.m. on Thursday before all the votes are tallied and the winner receives the official certificate of being elected to the country's top political office later in the day.
The prediction, meanwhile, comes as both Saenuri and the Democratic United Party (DUP) are urging their supporters to vote Wednesday.
Experts said because the two candidates are running neck-and-neck, turnout of loyalists will likely decide who wins the election.
In the past, higher turnout numbers favored the liberal camp, while a lower tally resulted in a win for conservatives.
This is because the older generation who generally back up conservative candidates in elections have a higher turnout rate than young people. Moreover, with the steady aging of South Korea's population, there are roughly 500,000 more voters over 50 than those in their 20s and 30s.
Reflecting this, DUP officials hinted the final turnout needs to exceed 74 percent in order for Moon to have a good chance at winning the race. (Yonhap)