A Hong Kong court has jailed a repeat sex offender for four months over an indecent assault of a teenage girl on a double-decker bus two months ago.

William Wu, 65, was sentenced at the West Kowloon Magistrates’ Courts on Tuesday, according to local media. He previously pleaded guilty to one count of indecent assault.

In June, the hardware store salesman was caught on camera groping a Secondary Form three student from behind on the upper deck of a bus travelling on Mong Kok Road near Sai Yeung Choi Street South.

The viral footage, captured by an onlooker from street level, showed the man reaching towards the underarm of the girl sitting in front of him. The teenager sought to distance herself from his touch, but Wu did not withdraw.

The defence told the court on Tuesday that Wu had three convictions for sexual offences, adding that he was willing to undergo psychological treatment.

The man had been detained since his arrest on June 8 and pleaded for leniency in sentencing. Principal Magistrate Don So sentenced the man to four months in jail.

Wu left the city following the indecent assault and was arrested on the same day as he re-entered from mainland China at the Hong Kong-Zhuhai-Macau Bridge.

At the time of the offence, the victim was going home after classes and did not immediately file a police report as she was frightened, according to police.

Hong Kong has proposed an overhaul of its sex crime laws, which were mostly introduced in the 1970s based on former legislation in England and Wales.

Authorities are seeking to introduce dozens of new offences, including provisions to protect mentally impaired people, as well as to expand the scope of rape, and redefine consent during sexual activity.

Rights groups including the Association Concerning Sexual Violence Against Women have urged the government to also introduce an offence related to persistent child sexual abuse, and to explicitly rule out the defence of “honest but mistaken belief” when it comes to sexual assault.