"This is not Photoshop, this is a real photo," said another.
"This isn't a scene from an upcoming dystopian sci-fi movie," said University of New York academic and futurist Amy Webb. "It's a street in Macau".
Mr Tsui's image was indeed taken from a street in the Chinese special administrative region of Macau. Once a Portuguese city-state, it is now one of the world's largest gambling hubs.
The alien spaceship bearing down on the street is actually the 47-floor Grand Lisboa casino built in 2008 and designed by architects Dennis Lau and Ng Chun Man.
Its bizarre shape, bulging at the base and splaying out as it rises, is said to have been inspired by the feathered head dresses worn at Rio de Janeiro's Mardi Gras festival, an architectural nod to Macau's Portuguese-influenced culture.
The plumes are also reminiscent of a blossoming lotus flower that can be found on the Macanese flag.
Photographed in its usual context, surrounded by other skyscrapers, the Grand Lisboa is a sight to behold. But in Mr Tsui's image it seems to take on an extraterrestrial form.
He told news.com.au he loved photographing "urbanscapes".
"The spot of this photo was not in my original plan. It was just a coincidence that the street was next to the hotel where I was residing."
The contrast of the casino, with the street full of old buildings, immediately attracted Mr Tsui.
"I took quite a number of (photographs) from different angles and distances from the street, and I liked this the most. It was captured using a tele [sic] lens to create that special effect of an alien ship invading the place." Hence why he titled the image "The Invasion".
But Mr Tsui said there was a further meaning to the image: "It can also be an underlying message that whether some people would think city modernisation is like an invasion to the original facets of an area."
Since Macau's rebirth as a gambling mecca, many of the historic colonial buildings have had to jostle for space and prominence with the new and glittering cathedrals to betting.
"The Invasion" came to widespread attention when it was submitted to National Geographic's Your Shot online photography community.
"I was quite amazed that this photo raised so much interest on the net, particularly in the US and Japan," he said.
"For most, they were attracted by the cyberpunk and surreal feel. Some debated whether it is a photoshopped image."
Despite the positive reaction to the image, Mr Tsui said photography was yet to be his full time job.
"My initials are PT, which can also stand for part time, so I describe myself as part time — an urban photographer."
