The same-sex marriage postal survey has caused further controversy after it was revealed that shining a torch through the envelope can reveal individual votes.
A photo was shared on social media showing the ticked 'no' box illuminated after held under light.
"So we have wasted $122 million on a survey where a torch can reveal the answer through the reply envelope it came with," the person who posted the photo wrote.
"Any angry postal worker with a vendetta against the opposing side can go through and remove votes as they see fit. Bravo government."
The Facebook post was later shared on Reddit and many voiced their concerns.
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"This feels inexcusable. If someone feels strongly enough about it and has the resources (say they work for Aus post) they could filter letters with yes or no votes depending on their own feelings on it," one person wrote.
"I received mine today and can confirm that even a small light like the one on my phone can easily see which box is ticked," another said.
The ABS began sending out the same-sex marriage vote slips on Tuesday.
In a statement to 7 News Online, an ABS spokesperson said it does not consider "shining torches through survey envelopes" to be a security or fraud concern.
"The survey form has no visible identifying information such as a name or address. This ensures that anyone with access to a completed survey form is unable to identify the respondent," the statement read.
"The ABS has used envelopes manufactured with a security lining printed on the inside.
"Theft or tampering with mail is a Commonwealth offence and carries serious penalties. Under Divisions 2 and 3 of the Australian Postal Corporation Act 1989, examining the contents of mail by someone who is not approved to do so is punishable by imprisonment for a term up to 2 years."