A 12-year-old girl was indecently assaulted by a man in the grounds of Macarthur Girls High School at midday on Monday.

Police said the girl had seen a man acting suspiciously near some garbage bins in the school grounds and had attempted to run away when the man knocked her to the ground and indecently assaulted her.

The alleged offender left the area and the girl sought assistance from other students. The girl was taken to Westmead Hospital for treatment.

Police spent the rest of the afternoon examining the area and blocked off Stewart St and Thomas St which both border the school.

Detectives are seeking public assistance in finding the offender who is described as being in his mid-30s, of white/European appearance, more than 170cm tall with an average build, brown medium-length spiky hair and long nose. He was last seen wearing a grey jumper and dark blue jeans and had a black mask concealing his eyes.

Anyone who witnessed the incident or has any information that might assist investigating officers is urged to contact Parramatta police station via Crime Stoppers on 1800 333 000.