Including a legal appointment for a 16(1) appointment letters

Ah, well spotted! Yes, you are quite correct, there is also another mandatory appointment, the 16(1) appointment, which refers to the head of the organisation. While you can make this appointment in writing, it is not really necessary, since the OHS Act makes this appointment compulsory in its definition:

16 (1) - Every chief executive officer shall as far as reasonably practicable ensure that the duties of his employer as contemplated in this Act, are properly discharged.

This statement makes no reference to appointing, rather it says -shall properly discharge-. This means that the head of the organisation is automatically made responsible for the overall health and safety of the company, whether they be the CEO or MD.