Often greywater is mixed with blackwater by the time it exits the slab, making direct garden re-use inadvisable for health and environmental reasons. Housing construction regulations could be changed to make mandatory the separation of greywater and blackwater plumbing inside floor slabs so that greywater can be accessed for direct re-use if desired. Direct greywater re-use for garden irrigation is currently illegal in all Australian states. Regulations are set by conservative state health departments whose main concern is the perceived public health risks associated with greywater re-use. This fear seems to be unjustified based on greywater health risk research in Australia and America.
Regulators are also concerned with high nutrient loads in greywater.
Research in Australia and America shows this to be unfounded in households which do not use phosphorus-containing detergents. Local councils can choose to override state regulations, and this has occurred in some unsewered areas of Australia. Encouragingly, direct greywater re-use for garden irrigation is now being examined by some Australian water authorities as an option for reducing fresh water demands in rural and urban areas. In January 1996, the Queensland Department of Primary Industries released a Policy Options Paper on the Use of Greywater, which examines the potential for regulated re-use of greywater in sewered and unsewered areas of Queensland. Diseases are potentially present in used house water only where occupants are ill.