Police said the 17-year-old was walking along Cunningham Street in Fairfield when a tattooed man armed with a gun confronted her, about 11.15am.

The man demanded the girl's mobile phone, then indecently assaulted her before running away.

The girl reported the incident to her school a short time later, police said.

Her attacker was described as being aged in his early 20s, of Asian appearance, about 165 centimetres tall, with a chubby face, brown eyes and black hair worn in a flat-top style and shaved at the sides.

He also had a dragon tattooed on his left shoulder.

He was wearing black clothes and white shoes at the time of the attack.

Anyone who saw the incident or has any information that could help police should call Crime Stoppers on 1800 333 000.