Elections Ontario said 566,845 people cast ballots during the seven days of advance polling from May 31 through last Friday.
That's a six per cent drop from the 603,785 electors who cast ballots in advance during the 2011 provincial election, which had a record low overall turnout of just 48 per cent.
One key difference: There were 10 days of advance polls in 2011, compared with just seven this year.
Pollster Bruce Anderson of Abacus Data said his firm's research has found undecided voters are making their decision more and more at the last minute, which might explain the lower numbers of people voting in advance.
"We've seen patterns lately where people are making their decisions on the final day, and in some cases even in the final hours," he said. "It may just be the case that people won't decide until they feel it's absolutely essential to decide."
Lower turnout in advance polls doesn't always translate to reduced turnout overall. The Quebec election in April saw a 16 per cent jump in advance-poll turnout, but overall fewer people showed up to vote.
If the overall trend continues downward in Ontario, it could rival the lowest recorded voter turnout in Canadian history for a provincial or federal election — the 40.6 per cent nadir in the 2008 Alberta election.
In recent years, Newfoundland and Labrador, P.E.I. and Manitoba have also seen record or near-record lows in provincial election turnout.
Elections Ontario pledged after the last election to conduct an analysis to determine why so many people didn't cast a ballot.