Search on for Sydney child attacker

A man has allegedly indecently assaulted a six-year-old girl outside a Sydney unit block, with police fearing he will strike again.

The attack appears opportunistic and was only stopped when residents of the Cabramatta block yelled at the man from their windows prompting him to flee.

Witnesses have told police the man approached the girl, who was not physically harmed, as she rode her bike within the complex's grounds around 3pm on Monday.

He then pushed her towards the rear of the block and allegedly performed an indecent act.

Acting Superintendent Craig Ireland said the attack appears to be an isolated incident.

"It's very frightening," he told reporters.

"It happened in daylight during the school holidays, a young girl on private property."

Police are searching for a man of Middle Eastern or Mediterranean appearance in his late 30s or early 40s, about 190cm tall with stubble on his face.

Anyone with information is asked to contact Crimestoppers on 1300 333 000.