The names and addresses of some 'silent' electors were available online for almost a decade in an accidental data breach affecting about 21,000 Melbourne voters.
The Victorian Electoral Commission in May uncovered the leak, which only became public on Thursday.
The breach related to two wards in the 2005 Bayside City Council election and the file has since been deleted.
The Proportional Representation Society of Australia, an electoral reform organisation, accidentally uploaded the personal information which could be found via search engines, secretary Geoffrey Goode told AAP.
It could not be accessed directly on the society's website.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"We didn't invite people to view it, we didn't even know it was there," Mr Goode said on Thursday.
A Local Government Investigations and Compliance Inspectorate probe revealed the voter leak occurred prior to 2009.
Electoral information is publicly available but legally cannot be published.
Of the thousands of compromised voters, 28 residents have since become silent voters, being granted anonymity on the electoral roll for privacy or security reasons.
"The VEC has notified the people and they have decided not to take any action," Mr Goode said.
"Obviously we're sorry, we regret the mistake."
No silent voters at the time of the 2005 Bayside election had their information distributed online.
Mr Goode believes the roll may have been published in an "inadvertent" bulk-file upload.
The breach will not be prosecuted because it occurred before the inspectorate's establishment and a three-year statute of limitations has expired, a statement from the LGICI said.
