‘After the statutory audit has been completed a fraud has been detected at the office of the auditee.’ What is your defence as an auditor?

There is no "defence". The responsibility to prevent and detect fraud lies with management. The auditor cannot be held responsible for detecting the fraud. The auditor however does have the responsibility to report it to the appropriate authority. The procedure to report it will depend on the prevailing legislation. The AFS will also need to be qualified and include the details and implications of the fraud. 

Management is responsible for establishing and maintaining the control environment. Auditors plays a role in a system of internal controls by performing  and making recommendations for improved controls. Furthermore, every employee plays a role in either strengthening or  weakening in the institution's internal control systems. Therefore need to be aware of the concept and purpose of internal controls

Auditors do not have a responsibility to prevent and detect fraud, it is management responsibility to do that by putting up strong and effective controls.