Greywater can carry large amounts of disease-causing microbes and should be used on gardens with caution, a Queensland University of Technology microbiologist has warned.

Dr Megan Hargreaves, of QUT's Faculty of Science, said that in the rush to respond to the water crisis some safety considerations for greywater had not been well communicated to the public.

"Greywater from the bathroom or laundry is full of organisms washed off our bodies," Dr Hargreaves said.

"These organisms are normal and safe in or on your body, but when they are in concentration and exposed to other areas they can cause gastro-enteritis and skin or respiratory infections.

"You could find yourself quite ill if you eat, say, salad leaves that have greywater on them and haven't been thoroughly washed, or if greywater gets into a cut it can become infected.

"If you spray it using a hose, then you could breathe in contaminated aerosols and get a respiratory infection."

Dr Hargreaves said the safest way to reuse greywater on the garden was with subsurface irrigation which prevented plants and people being exposed to the potentially harmful microbes.

"Delivering greywater to plants from an underground system is the safest method because it avoids exposure of people to micro-organisms which are filtered out by the soil," she said.

"Even hand watering around plants has its hazards because the organisms can multiply in greywater puddles. It is important that children and pets are kept away from any greywater puddles."

She said greywater should not be stored longer than 24 hours so that micro-organisms cannot build up in it.

"Greywater systems designed for the home usually include some form of treatment, such as filtering," Dr Hargreaves said.

"Homeowners who are having greywater systems installed should ensure the filters are easily checked and cheaply replaced before they buy as filters tend to clog very quickly with hair and other debris."

Dr Hargreaves said there had been a lot of pressure on local governments to facilitate greywater use by homeowners quickly and many councils were now developing guidelines for safe usage of greywater.

Safety tips for greywater use:

• always wash your hands after using greywater
• do not spray greywater
• install an irrigation system that runs under the soil
• do not let greywater pool on the ground
• install an easily maintained filter
• do not store greywater longer than 24 hours.