The idea of having the head of a the ACT police force as a statutory appointment under ACT legislation is merely one of symbolism. At present the nominal head of ACT policing is an officer of the Australian Federal Police appointed by the Commissioner of the AFP who is answerable to the Federal Government. In fact the appointment is made with consultation with the ACT Government. If the office were to become an ACT statutory office it would not be filled without consultation with the Federal Government, so in practice nothing much would change _ both Governments would have an effective veto.
Some see having a separate statutory head of ACT policing as the precursor of a separate police force. The ACT used to have its own police force answerable to a Federal Minister. However, since the creation of a separate Australian Federal Police, the ACT force has been subsumed – to the advantage of the people of the ACT. The AFP provides better career opportunities and the ACT gets the advantage of officers moving between ACT and federal duties. The ACT also gets all the economies of scale associated with a bigger force.
All of the experience in smaller state forces suggests that the ACT is doing pretty well out of present arrangements. The ACT is not a state and never will be. In some areas it will always be inextricably bound up with the national government, so there is no need to separate the police forces.