I think you better be guided by Section 60F of the Employment Act as quoted below:
"60F. Sick leave.
(1) An employee shall, after examination at the expense of the employer:
(a) by a registered medical practitioner duly appointed by the employer; or
(b) if no such medical practitioner is appointed or, if having regard to the nature or circumstances of the illness, the services of the medical practitioner so appointed are not obtainable within a reasonable time or distance, by any other registered medical practitioner or by a medical officer;
be entitled to paid sick leave....."
So, if they have an appointed panel doctor, you have to see the panel doctor in order to qualify for paid sick leave as in (a). The exceptions are stated in (b) where the clinic is too far away or having an illness that requires urgent treatment. Should disputes occur, you may have to refer to the Labour Department.