Sam Smith is backing Same Sex Marriage in Australia

Sam Smith is donating a third of the expected sales for his concert at the Sydney Opera House next year to Australia's same-sex marriage campaign, according to a report by Eagle Radio News in Australia.

The Oscar-winning singer is one of a number of stars urging people to say a big fat YES to same-sex marriage, and last month he shared a post on Instagram about the vote, telling Australians: "You have a special, special place in my heart."

He has now offered to donate proceeds from the show ahead of the Australian same sex marriage vote.

Tickets for the show on January 16 went on sale on Monday morning, starting at $79.90. With a capacity of just under 2,000, it means the donation could reach the $50,000 mark.

The Opera House confirmed the concert sold out within hours of going on sale.

Australian Marriage Equality co-chair Janine Middleton said: "Sam knows all too well about the challenges faced by LGBTI Australians and his generosity towards them will help us achieve fairness and equality for all.

"Sam doesn't need to do this, he isn't Australian but he understands what it feels like to be in a relationship that isn't considered equal. Soon we hope to realise the equality he enjoys at home in the United Kingdom will soon be a reality here in Australia."

Sam Smith described Australia as a "home away from home" when he announced the gig earlier in October.

The Australian marriage equality vote closes on November 7, with the result due to be announced on November 15.