Bird of the Year rules allow one vote per person, so all but one of the fraudulent votes have been deleted.
"We're not mad, just impressed that someone cares enough about New Zealand's native birds to rig the competition," campaign co-ordinator Kimberley Collins said.
"We suspect their plan was to sneakily increase the heron's numbers by a few hundred each night while we were all sleeping. Thank goodness Yvan was watching."
Forest and Bird didn't expect the culprit to come forward, but hoped the weight of their conscience will prompt them to make a donation to help protect native birds.
"Although the white-faced heron is doing okay, many of our native birds are in crisis," Collins said.
"Sixty-eight per cent of our birds are in trouble and one in three are at risk of becoming extinct Forest and Bird has also taken every security precaution to ensure it does not happen again."
Bird of the Year is one of Forest and Bird's most popular annual events.
It started in 2005 and aims to raise awareness of New Zealand's unique native birds and the threats they face by asking people to vote for their favourite species.
Voting closes at 5pm on Monday, October 23, and the winner will be announced the next morning on RNZ's Morning Report.
• A previous Herald story incorrectly stated voting closes on October 30.