Police on Saturday said they arrested a man over an alleged sexual assault case involving an underage girl that took place more than three decades ago.

The suspected abuse came to light last month when a 44-year-old woman filed a report to the force, alleging she was molested by a man between 1993 and 1994.

The man, 66, was picked up by officers in Fanling on Friday over alleged unlawful sexual conduct with a minor following a detailed investigation, according to police.

The man has since been released on bail.

"Police have all along had zero tolerance towards sex crimes and will follow up seriously regardless of when the case happens," the force said.