It is common practice to use temporary speed zones on unsealed roads and Australian Standard 1742.3 Traffic control for works on roads allows their use (however, this is not mandated due to the low volume of traffic and economic considerations which shall be based on a risk assessment, see section 4.5 of AS1742.3).
It is also worth noting that it is common practice to speed zone unsealed sections of roads for side tracks and when the sealed surface has been removed due to road resurfacing.
View Sample Diagrams, to show how a work site may be set up on an unsealed road.
When speed zoning unsealed roads it is important to ensure the surface condition is inspected and maintained to an acceptable standard for the speed limit implemented. The END speed limit signs shall be used when terminating the temporary speeds.