If expertise in a field other than accounting or auditing is necessary to obtain sufficient appropriate audit evidence, an auditor may need to use the work of an auditor’s expert. The need for special expertise may include such matters as:
• The valuation of complex financial instruments, land and buildings, plant and machinery, jewellery, works of art, antiques, intangible assets, and business combinations and conducting impairment reviews
• The actuarial calculation of liabilities associated with insurance contracts or employee benefit plans
• The estimation of oil and gas reserves
• The valuation of environmental liabilities, and site clean-up costs
• The interpretation of contracts, laws, and regulations
• The analysis of complex or unusual tax compliance issues

Required:
How does an auditor determine the need to use an expert when developing the overall audit strategy? What additional work does an auditor need to perform if planning to use an expert’s work as audit evidence to form an audit opinion?