Two domestic helpers were arrested on Tuesday on suspicion of child abuse after medical staff said a four-month baby boy reportedly showed symptoms of Shaken Baby Syndrome.

Police received reports on Sunday from Prince of Wales Hospital that the baby had suspicious injuries and bleeding in his brain and back of the eye.

Upon investigation, officers found bruises on the baby and his 16-month-old sibling.

Police also looked through surveillance camera footage within the household in Tsiu Keng Village, Sheung Shui.

Officers will soon press charges against one of the suspects, 28, for ill-treatment or neglect by those in charge of a child or young person.

She will appear before a magistrate in Fanling on Thursday.

The other woman, 45, is being held for investigations.

Both children have been admitted to hospital for treatment, with the four-month-old in a critical state after undergoing brain surgery.

Police said they will pay close attention to their condition.

Senior inspector Kwok Sze-wai said the quick reporting of the incident by medical staff allowed police to take action in time.

"I would like to thank the medical staff who called officers on the case. Police were able to intervene timely and carry out subsequent investigations due to their early identification of child abuse signs. This shows the legislative intent of the Mandatory Reporting of Child Abuse Ordinance that took effect on Tuesday," Kwok said at a press briefing.

The new regulations require 25 categories of professionals, including doctors, social workers, and teachers, to report suspected serious maltreatment cases.

Kwok said everyone should play a part in child protection work and report any suspected cases to the police or the Social Welfare Department.