A council may need outside assistance in the preparation of accounts, depending on the skills and inclinations of the RFO.
However, the procedure as outlined seems inadequate. The council must approve the accounts before the internal auditor can act, and this is evidently not happening. Also, if the internal auditor is preparing the accounts, I would doubt whether they could then be counted as independent of the council. Their involvement in accounts preparation would seem to prevent them from acting as internal auditor.
There is also a duty on councillors to ensure that there are adequate financial controls in place, and it could certainly be argued that the process described does not meet that requirement.