You are conducting a year end audit for St Vincent's Hospital, a private hospital in Sydney. Following your initial review of the business you have identified the following significant risks.

Payroll expense

Due to a shortage of staff, St Vincent's had employed a significant number of casual nurses, administrative and cleaning staff to support their full-time staff. Huge overtime hours are common especially on the weekends and night shifts.  

Determine the key assertion at risk
Describe a practical detective internal control that could be implement in relation to the risk