I could be wrong about this, but to the best of my knowledge there are no specific qualifications required for an internal auditor. There are requirements for the external auditor, but local councils have the external auditor appointed for them and have no choice in the matter.
Although financial knowledge is desirable, it is at least as important that the internal auditor understands local council law and procedures, as they are investigating whether expenditures fall within the powers of the council and have been properly decided upon by lawful decisions of the council.
Some council clerks also act as internal auditor - obviously for other councils than those for which they work. Likewise some retired clerks. Otherwise, the most likely source is a local accountant who is willing to acquire a working knowledge of local councils.