Same-sex marriage postal surveys arrived in letterboxes around the country this week and some Australians have been quick to show off their votes on social media.
But the ABS has asked people to avoid posting pictures of their ballot online to safeguard privacy and security.
The barcode on the bottom right corner of each ballot paper is linked to the corresponding voter.
The Australian same-sex marriage postal ballot with the unique barcode.
This allows each participant to only vote once during the postal survey.
The ABS is concerned people will be able to replicate the barcode from a photo uploaded online and then post a false response.
If the faulty vote reaches processors before the actual owner's paper, it would be counted instead as every barcode can only be registered once.
The ABS have played down the concern of tampering, but have warned to keep all voting secure to safeguard responses.
"The ABS discourages people from posting images of their Australian Marriage Law Postal Survey materials online in order to safeguard the privacy and security of their response," an ABS spokesperson told SBS World News.
Voters also raised privacy concerns on Wednesday when photos of the responses on the ballot were easily recognisable under torch light.
A photo posted on Facebook and Reddit claimed to easily show a voter's decision when a light was directed through the envelope.
A Reddit thread started with claims you could see the vote on a ballot if you shined a torch on the envelope.