He looked like an easy target: a medical student sitting alone on a late-night train.

The three men sidled up to the German student and told him to hand over his wallet. When he refused, they followed him off the train, trailing him as he walked past the hotel and pizza joint on Bringelly Road, Kingswood, in the city's west.

When the student, who has asked not to be identified, turned down a dimly lit alley his pursuers thought their moment had come.

The men tripped the 27-year-old student, kicking him as he lay on the ground and grabbing his mobile phone and iPod.

What the assailants did not realise was that they were standing outside Ninja Senshi Ryu - western Sydney's ninja warrior school.

They also failed to notice a ninja, Nathan Smith, standing in the shadows outside the dojo. Mr Smith immediately alerted his sensei, or teacher.

Kaylan Soto, a sensei with 30 years' Ninjutsu training, and three of his students raced out of the dojo towards the startled attackers. All five crusaders were clad in the ninja's traditional, all black uniform.

Mr Soto said it took the three assailants a few moments to realise what was going on. When they did, they shot off. "You should have seen their faces when they saw us in ninja gear coming towards them," he said.