A children's rights campaigner on Tuesday urged the authorities to be thorough and humane as they investigate the case of a 12-year-old boy who was abandoned at a Hong Kong hospital.

The police have applied to the Juvenile Court for a care or protection order for the boy, who is staying at the Tuen Mun Children and Juvenile Home after being found wandering around Kwong Wah Hospital on Friday. He told authorities that his mother had returned to the mainland.

Speaking on RTHK's Hong Kong Today, Priscilla Lui, former director of Against Child Abuse, said seeking a protection order was the correct step at this point.

"The 12-year-old boy is in desperate need of care and protection as he was, quote and unquote, abandoned by his parent," Lui told RTHK's Samantha Butler.

"So I think that's a very important move for the present moment instead of right away allowing the child to be returned to the mainland.

"I think the situation needs to be made very clear. There needs to be a very thorough and in-depth investigation by both parties" in Hong Kong and on the mainland, she added.

Lui noted that child abandonment is a criminal offence that can carry a sentence of up to ten years in prison.

The Social Welfare Department said on Monday that it was looking into the case. Police officers are said to have been in touch with the mother, who brought the boy to Hong Kong from Jiangxi, and had expressed a willingness to return to collect him.

On Sunday, Chief Executive John Lee called on the family of the abandoned boy to come back for him, stating that such actions cannot be tolerated in the caring society he aims to build.