Most audit schemes around the world include an internal audit. However, many people are not totally clear on the concept. When I asked whether a company has an internal audit program, the answer is often yes, and the company proceeds to show me a GMP checklist it conducts once a month or so. This is an internal audit of a sort, but it is not what ISO 22000 and most of the Global Food Safety Initiative-approved schemes require.
The internal audit is a crucial element in the verification process. But the mantra for all processors should be develop, document, implement, maintain, update and seek to continually improve their food safety management system. The roles of the internal audit are to verify the system is working as designed (maintenance), see if it is operating as effectively as it should (updating) and provide a base line for improving it (continual improvement).