The 27-year-old German had just gotten off a train when three men followed and attacked him in a Kingswood alley, New South Wales police told the Australian Broadcasting Corp (ABC).
But the assault happened to take place just behind the Ninja Senshi Ryu warrior school, where a person leaving a martial arts class witnessed the scuffle and called for backup from a teacher and three other pupils, the broadcaster said.
"We looked around to see what was happening and there were three blokes on this guy just kicking him and punching him in the head," Ninja sensei Kaylan Soto told ABC.
"We started running towards them and they took off. They would have seen five of us in ninja gear...all in black with our belts on, running toward them,” he added.
ABC reported that police later picked up 16-year-old and 20-year-old suspects. Both men have been charged with the robbery, but officers are still looking for a third suspect between 15 and 17-years-old.
"I think they're probably still running if I'm not mistaken," Soto told ABC.
According to news agency AP, the German victim sustained minor injuries, and his assailants managed to make off with his mobile phone and iPod.