Some Australians have expressed their concerns about the privacy of the same-sex marriage postal survey, after photos shared widely on social media appeared to expose a trick that could reveal the responses inside sealed envelopes.

A photo that was initially shared on Facebook showed how shining a torch light over the reply-paid envelope included with the survey could reveal the marked Yes or No box on the form inside.

The reddit post has more than 100 replies. ( Supplied: reddit )

"So we have wasted $122 million on a survey where a torch can reveal the answer through the reply envelope it came with," the user wrote.

"So any postal worker with a vendetta against the opposing side can go through and remove votes as they see fit. (Or workmate if you post from work). Bravo government."

The ABC conducted a test and found the torch light did indeed reveal a clear mark in the Yes or No box.

The Facebook post was later shared on reddit, where users discussed their concerns about the privacy of the survey.

"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.

"This absolutely does not feel like an OK thing for the Government to not have ensured."

The ABS began sending out same-sex marriage surveys on Tuesday, and expects to have all forms mailed out by September 25.

Key dates in SSM postal survey: August 24 — the final day to register with the AEC if you want to take part in the survey

September 12 — survey forms start being sent out

September 25 — all forms are expected to have been sent

October 27 — forms are strongly encouraged to be returned by this date

November 7 — the final deadline to return surveys

November 15 — results are released

People have until 6:00pm on November 7 to return their completed surveys by post.

Some advocates for change had taken to social media to encourage people to put glitter in their envelopes.

But the ABS took a dim view of that suggestion, making it clear that any envelopes with glitter in them would simply be binned.

"Any extraneous material inserted in the envelope with the survey will be destroyed. This could also contaminate processing machinery or result in the survey also being destroyed and not processed," it said.

A spokesperson for Mathias Cormann, Acting Special Minister of State, said Australia Post had undertaken a thorough review ahead of the survey and would have additional security measures in place during the process.

"Australia Post are also working closely with the authorities to maintain the integrity of the mail network," she said.

An Australia Post spokesperson said it was a crime to tamper with mail.

"We have strong security measures in place throughout our national network," the spokesperson said.

"We have undertaken a thorough review ahead of delivering the survey and will have additional security measures in place during the process. We are also working closely with the authorities to maintain the integrity of the mail network."