The external auditor is chosen by the Government. The internal auditor is chosen by the Council. The internal auditor goes through all of the records and checks not just the figures, but policies and procedures. The external auditor sees a few numbers on the AGAR, the Council's self-certification that it hasn't broken any rules, and the internal auditor's certificate (which is different from the audit report.)
There are no minimum qualifications for internal auditors and too many Councils choose their best friends.