It was Leach-McGill's first tweet in 15 months, since he retweeted a pro-Safe Schools tweet on March 3, 2016. His account has only tweeted 23 times, and has not responded to requests to comment.
The tweet went viral on Monday morning, with ABC News, Huffington Post, Junkee and LGBTI media in Australia all reporting on the alleged poster as part of coverage of the upcoming postal survey on same-sex marriage.
The opposition Labor leader Bill Shorten went on to condemn the poster, and prime minister Malcolm Turnbull suggested LGBTI Australians confronted by it should be hugged.
BuzzFeed News has attempted to trace the poster's origins.
Using a reverse image search, the earliest appearance of the poster in question appears to be on Neo-Nazi forum Iron March in March this year.
Variations of the poster on other sites have the Iron March website URL in the bottom left-hand corner. The URL appears to be blacked-out in the photo of the poster reportedly seen in Chinatown.
A US-based user claiming credit for creating the poster has tweeted his joy at seeing the poster being used in Australia.
Leach-McGill's snap of the poster appears to be the only one that has been posted publicly on social media, and no other photos of the poster have so far surfaced online.
Despite all the attention, none of the news outlets that reported on the poster have independently verified that it was ever on display in Melbourne.
When BuzzFeed News spoke to Melbourne City Council on Monday afternoon and Tuesday morning, a spokesperson said they had not seen the poster, or had any reports of the poster being plastered anywhere in Melbourne.
BuzzFeed News attempted to contact several people who either claimed to have seen the poster, or to have covered it up with pro-marriage equality posters, but none have been able to produce evidence that the poster itself has been seen in Melbourne.
One of the lead campaigners in the “no” side, Australian Christian Lobby managing director Lyle Shelton, was asked about the poster on Ten's The Project on Monday and claimed it was a false flag operation.
“That obviously doesn't come from us, nor would it,” he said, suggesting that The Project's Waleed Aly's argument that the “no” side wasn't being respectful might be why the poster was put up.
“I don't know who put that poster up…I suspect it might have been put up by people trying to prove the point you are making.”
The poster cites a study by Catholic priest and sociology professor Donald Sullins from the Catholic University of America on same-sex parenting and the effects on the children of those relationships.
The study is frequently cited by anti-marriage equality groups such as Marriage Alliance in arguing against same-sex marriage, but the study did not examine the children of same-sex couples who were married. The study also does not examine how long the child lived with a same-sex couple.
The group appears to be based in Brisbane, and has just over 500 likes on Facebook. On the group’s Facebook page, the group took credit for the posters.
“The destruction of Traditional Marriage, the forced mixing of cultures into our nations, the consistent propaganda perpetuated by inflated facts or down right lies has GOT TO STOP,” a post states.
There is currently a Senate committee looking into the postal survey on same-sex marriage. Chair of the committee, Labor senator Jenny McAllister, has asked for such material to be sent to the committee.
Do you have questions about Australia's upcoming postal survey on same-sex marriage? Same. Here's a list of everything you need to know, and more.