A household is still a workplace for some individuals, so a risk assessment, even if it is very basic, would have to be in place. As long as it identifies all the hazards the employee is exposed to and lists the control measures which have been implemented, that should be fine. It should not only include domestic workers inside the house, but outside the house as well, such as gardeners. The OHS Act does not differentiate between informal and formal work, as long as there is an employer/employee relationship and they meet the definition of the two.