More than a third of advance voters chose no alternative.
The advance votes were in line with recent polls, which have consistently shown a preference to retain MMP. Final results are due by December 10.
Vote for Change spokesman Jordan Williams said it was always going to be an uphill battle to ditch MMP.
National made a strategic decision not to debate the referendum, which allowed the unions, Greens and Labour to frame the debate, he said.
"Unfortunately, because it was only a half-cooked debate, we didn't get there in the end.
"Certainly as the campaign progressed, support for MMP reduced and it appears that it's got just a little bit less than what it got last time, which is hardly an overwhelming endorsement but it's a majority."
If more than 50 per cent of people had voted to ditch MMP, Parliament would have decided whether to hold another referendum in 2014 to choose between MMP and the alternative voting system which got the most support.
But with the system seemingly safe, the Electoral Commission will conduct a review next year to consider if changes should be made to how it works.
The review will be reported back to the Government by October 31.
Mr Williams said any changes to MMP should be put to voters rather than being decided on by politicians, a situation he compares to "foxes guarding the henhouse".
"A voting system shouldn't be able to be easily manipulated by our politicians. New Zealand voters should have the final say," he said.
Keep MMP spokeswoman Sandra Grey said she was encouraged by the advance votes and was hopeful of a similar final result.
"It does show what the polls have been saying to us for the last couple of weeks, that there just wasn't the appetite for change and people did actually want to retain MMP."
She disagreed that the debate was half-cooked, saying that while the five-week campaign period meant the referendum took a back seat to the election debate, there had been a lot of press coverage.APNZ
