The Electoral Commission is certain that Votesafe anti-euthanasia material was not included in the EasyVote packs sent out to 3.3 million voters.
Photo: RNZ
The commission knows of 24 people across New Zealand who received the flyer asking them to vote no in the referendum in their official envelope.
Chief electoral officer Alicia Wright said they had investigated how it happened but had ruled out anything being added through the packaging machines.
"Every scenario for the brochure to be inserted into the pack or swapped out with other material has been looked at and eliminated."
She said when lots of election material was delivered, people thought the items had arrived together.
The voting packs are compiled at NZ Post sites in Auckland and Christchurch using a fully automated process.
The commission has confirmed that NZ Post had run various tests and determined that no additional material could have entered the process without detection by its machines.
"The machines are finely calibrated and reject any overweight items. These tests, along with our other enquiries, have led us to conclude that only material meant to be in the packs, went into the packs," Wright said.
She said there was no evidence of foul play or tampering, but she had asked the police to investigate further.
Earlier this week Safer Future Charitable Trust, that is running the Votesafe campaign - a group that opposes the End of Life Choice Act - said the flyers were printed in the same facility as the flyers for the Electoral Commission.