When it comes to internal audits, they should always be performed by your internal audit group. There a several reasons why, but the two most compelling are:
- The need to be independent and act in the organization's best interests vs. a line of business. Auditors must be independent as determined by the standards for independence in the AICPA Code of Professional Conduct.
- Auditors usually have specific licenses and certifications that qualify them to perform audits. Typically that is in the form of a CIA certification (Certified Internal Auditor), and many auditors also have CPA (Certified Public Accountant) Licences.
I hope that information is helpful, but I would love to hear from other members as well.