Bus services charging separate fares, or any free service operating for more than two weeks a year, are illegal without prior approval, the Transport Department says.

The warning comes as non-franchised bus services have recently been running between the urban area and San Tin Public Transport Interchange, operated under the guise of a "members-only service".

The service recruited passengers by inviting them to join as members, and charged each a fare of about $20 a trip. The department said it will work with Police to crack down on the illegal operation.

Those contravening the Public Bus Services Ordinance or the Road Traffic Ordinance are liable to fines and imprisonment, and having their licences terminated.

So far this year the department has initiated about 50 inquiries, and suspended passenger service and vehicle licences for 30 buses registered under 15 operators.