NZICA members still have the chance to cast their vote in person at a special general meeting on Friday, when the final result will be confirmed.
Craig Norgate, chief executive of NZICA, said yesterday marked a significant day in the body's history.
"We've been overwhelmed with the voter turnout at this point of over 16,000 which represents approximately 58 per cent of our membership."
The turnout was well above normal vote levels of 10-15 per cent, Norgate said.
According to the One New Institute website, 61 per cent of North Island members and 59 per cent of South Island accountants have voted so far.
A group of about 150 accountants, led by Hamilton accountant Bruce Sparrow, set up a website some months ago called 'Say No To One Institute'.
The group believes New Zealand and Australia are totally different countries in terms of taxation and politics, and merging the two makes no sense.
There is also a fear of losing sovereignty to a much larger Australian body, which has 73,000 members compared to NZICA's 33,000.
Between May and July, NZICA and ICAA carried out eight weeks of consultation by holding face-to-face roadshows, webinars, accepting submissions and by polling members.
Norgate claims the merger will result in lower fees and a more powerful organisation for Kiwi members.
If members give the green light, implementation of the so-called One New Institute is expected to begin next year.
The final result will be made public this Friday afternoon.