An Australian couple have been united with a "fairytale" photo of their engagement, after a tourist found the pair via social media.

US photographer Rob Lau captured Philo Botros' marriage proposal to Monica Gaid on Sydney Harbour last month.

The picture came to their notice after it was shared online by Mr Lau's friend.

Ms Gaid said discovering the picture-perfect shot was a "surreal" experience.

"I had no idea this photo even existed, until my friend saw it on Facebook. It was just not expected in any way, and now we have this amazing, literally one-of-a-kind photo," she told the BBC.

Mr Lau said he had been in Sydney for a short work trip that day and had wandered over to the scenic spot at Mrs Macquaries Chair for some last-minute sightseeing.

"As a hobbyist photographer whenever I travel I try to get a couple of sunset shots in," he said.

He was about to leave when a noise from the water's edge caught his attention.

"At that moment I saw a guy get down on his knees getting ready to propose, and I quickly pivoted my tripod and snapped a few shots," he said.

He captured the pair's silhouettes against a backdrop of the Sydney Opera House and Harbour Bridge in a wide panorama.

Ms Gaid said the noise that alerted Mr Lau had been the sound of her designated photographer tripping and falling into the water. As such, she thought they had missed out on photos of the moment.

After Mr Lau returned home to California, his friend shared the picture on Reddit and speculated that he could find the couple. Ms Gaid got in touch within two days.

She said she was grateful to Mr Lau for the photo which she described as something out of fairytale.

"It's so lovely to know there are good people out there, happy to go through with all that effort for the sake of doing something good and getting nothing in return," she said.