Hong Kong police are investigating an assault case against a secondary school student who allegedly attacked an Indonesian domestic worker on Wednesday.

At 11am, police received a report from school staff in To Kwa Wan, Kowloon that a woman had been attacked by a student, news website HK01.com reported.

Police initially thought they were dealing with an insane person, but when they arrived at the school, they arrested a 16-year-old boy for alleged assault occasioning actual bodily harm.

The student allegedly attacked the Indonesian woman, aged 44, using his hand and his mouth. She suffered an injury to her mouth and was taken to the Queen Elizabeth Hospital for medical treatment.

The report did not mention the reason behind the attack.

The crime investigation unit from the Kowloon City Police Station is investigating the case.