The 18-year-old is "incredibly frustrated" after the Australian Bureau of Statistics (ABS) told the HSC student he could not take part in the same-sex marriage postal survey because it can't verify his details.
Mr Cairnduff has two lesbian mothers and was eager to receive his ballot so he could vote so they could get married in front of friends and family in Australia.
The HSC student enrolled on the Australian Electorate Commission website this year on February 13, two days before his 18th birthday, to ensure he would be eligible to vote.
SBS World News can confirm Mr Cairnduff is enrolled to vote on the AEC website. He also said he voted in his local election this year.
But Mr Cairnduff did not receive a ballot by the ABS September 25 deadline for mail-outs.
The 18-year-old spent hours on the phone to ABS supervisors trying to determine why a ballot was not delivered to his address.
"We called up the ABS and they came back to us, after a long time speaking to a couple of supervisors, saying they couldn't find me on the database, but they could find me on the website [AEC]," Mr Cairnduff told SBS World News.
"They said call back when we will be handing back our second round of postal votes. After that long call, I called back on the 26th, and they said to me they didn't think I could vote because they couldn't find me on the database."
After enquiring further - in an email showed to SBS World News - the ABS replied claiming he was not eligible to vote in the same-sex marriage postal survey.
"Unfortunately we have not been able to verify your details against the Commonwealth Electoral Roll so cannot process your request at this time," it read.
Mr Cairnduff said he is frustrated he has been forced to put so much effort into fighting for his legal right to vote. He is also concerned other Australians might be discouraged if they are facing a similar situation.
"I am incredibly frustrated that I have had to put this much effort to essentially fight for my vote, in a process I frankly don't want to be doing anyway," Mr Cairnduff told SBS World News.
"I am frustrated, you see these people in the media and they speak about your family and what my family is and what it means... And they are going to have their right to vote, but I am not going to be able to vote on whether my mums can marry."
Patrick is hoping he will be able to see his mother, Annette Cairnduff, and her partner, Kylie Gwynne, marry for the second time in Australia.
The couple were married for a very brief time when same-sex marriage was legal in Canberra, before the Marriage Act was amended in 2004 to only recognise the union between a man and a woman.
Annette Cairnduff is also the co-founder of Just Equal, a human rights group inspiring Australians to respect equality for LGBTI Australians.
'This means the world to us'
Mr Cairnduff said this issue was of paramount importance to his family and urged Australians who may not have received a ballot to persist for their right to vote as it would be making a difference.
"If you're not in a family like mine, it is not going to directly affect you and it may be a bit more of an effort [to request a postal vote]," Mr Cairnduff said.
"But, for family's like mine this means the world to us to be able to marry. When I saw my parents get married in Canberra, to see them smile... and to see how happy we were that this day had finally come around, to us it's just so important."
Alex Greenwich Co-Chair of The Equality Campaign said they will be raising the concerns with the ABS.
"Every single survey in this postal vote counts, stories like this highlight just how important it is to return your survey," he said in a statement to SBS World News.
"We will continue to raise these concerns with the ABS and encourage everyone to return their YES votes."
The ABS confirmed to SBS World News they are investigating the issue and the AEC said they are unable to comment on an individual case for privacy reasons.
