my relative who is now working in Housing Department, told me that the Department encourages public to complain any flat abuse cases. They have a big team to investigate these cases. All you need to do is to give them the address of the unit and name of the tenant and ID card no, if any with any info you regard as useful. You can write the info on the special form which can be got from any estate office.
We should help to rectify this unfair phenomenon.