Not that long ago, Christopher Thé was making 30-40 pieces a night of his signature strawberry watermelon cake at Sydney's Black Star Pastry. "It was just another cake that we did," he says.

Nowdays, it's such a blockbuster dessert that Black Star Pastry can produce 4000 portions of it in a single night. More than a ton of watermelon is used every week to layer the cake and "supposedly this is the most Instagrammed cake in the world", he says.

Thé laughs and says he's not sure how you'd even prove that, but there's no doubt that it's overtaken people's social media feeds - and lining up for the cake at the Black Star Pastry outlets has become a ritual.

"A lot of people come from the airport, they set up with their cameras, they even bring their own lights, they post out to their friends. It's great," he says.

"We are starting to see several versions of it around the world, which I'm OK with, it's quite a compliment."

The cake - which is layered with strawberries and has a "red racing car stripe" of watermelon throughout - began life as a wedding cake for a friend.

"I just wanted to make it as romantic as possible," he says. The rose petals and the rosewater are Middle Eastern influences and the watermelon is like the dessert course at a Chinese restaurant - straight-up fruit as palate cleanser. When he first finished it, "I remember my wife saying, 'what are you doing?!'"

Clearly the chef knows what he's doing as he predicts that he's sold "six football fields worth of strawberry cake since we started" Black Star Pastry in 2008. "That's a whole lot of watermelons and strawberries!"