"Had to zoom in for a sec," one admitted, while other frenzied Efron fans tagged their friends to ask if "they could see it too."
Take a gander for yourself below.
The actor, who rose to fame in the High School Musical films, available to stream on Foxtel, has one of the most impressive physiques in Hollywood, often stripping down in his films to the delight of fans.
But his latest Instagram snap isn't the first time a topless Efron has left fans scratching their heads.
Earlier this year, during an appearance on Ellen, the star lifted his shirt to reveal his jacked six-pack, leading many to accuse his seemingly unattainable muscles of being "fake".
"I love him but those look airbrushed," one baffled user wrote on a clip shared by DeGeneres.
"It looks fake," another declared.
"Are they even real?" one person questioned.
Others suggested his tanned torso had been enhanced with makeup, saying he'd "contoured" before appearing on the show.
The actor regularly shares videos of his gruelling workouts to his 40.5 million followers.
Back in 2016, Efron revealed in a tweet exactly how he got so shredded for the Baywatch film, and it was intense.
Though not quite as intense as a Zac Efron fan thinking they've caught a glimpse of his bits.
