In some states, the legal requirement to wear a helmet on the road doesn’t apply on farm. Even so, you do have an obligation to employees under Workplace Health and Safety laws, and employees have an obligation too, to ensure that they act in safe ways. Quad bike accidents are one of the most common causes of death on farms, so you are really sensible to be clear about this with employees.
You can set a code of conduct that requires everyone to wear a helmet when on a bike or quad on the farm. This can be a requirement for employment (there are a couple of the bigger farms who have a ‘no helmet no bike’ or ‘no helmet no job’ policy).
You can download and write your own code of conduct and safety protocols for your farm. Visit the Health and safety risk management section for more information.
There is also some useful info about quadbike and vehicle safety at the Farmsafe site.