As external auditors, how can we rely on the work of internal auditors?

"Audit rules require auditors to obtain an understanding of a client's internal controls. As part of their responsibilities, an internal audit team often documents and assesses internal controls for their organization. In some cases, the external auditors can leverage this work in completion of their financial statement audit. In other words, they don't need to duplicate the work of the internal audit team. However, as external auditors we will often need to do our own transaction testing and therefore cannot rely on the internal audit team's work."