Multiple voting could be occurring in the same-sex marriage postal survey just days into the mailout to 16 million people.
Voters have boasted of returning multiple same-sex marriage survey forms after receiving mail addressed to previous residents.
A Victorian woman engaged to be married to her female partner tweeted: "I get SO much mail for previous people who lived in my house. Every damn one of these is being opened and sent back as yes."
A South Australian man tweeted: "Big ups to the seven people that haven't changed their enrolment info and their vote mail came to my house. My vote just evolved seven times hahaha."
However when contacted by AAP via email on Thursday afternoon, the man said he was joking.
"But if I did have seven envelopes - like I'm certain every sharehouse in Newtown will have - I'll probably vote yes for them," he said.
An ABS spokeswoman told AAP measures were in place to ensure the secrecy of each survey response and the integrity of the process.
"These measures will detect any attempted multiple responses associated with an individual eligible Australian with only the last valid response counted."
The ABS warned theft or tampering with mail is a criminal offence and carries serious penalties.
Examining the contents of mail by someone who is not approved to do so is punishable by up to two years in jail.