Three homes in Melbourne's west were surprised last month to find their drinking water had changed to a "shandy" of recycled sewer water and clean drinking water.
About 4000 properties in Werribee, Wyndham Vale and Manor Lakes were connected to "Class A" recycled water on November 17, for a City West Water project.
But three residential properties were incorrectly connected to the recycled water through their drinking water supply in West Wyndham, the water company said.
A City West Water spokeswoman said before the recycled water was connected to the properties, all customers were advised to be "diligent in checking their internal connections and to monitor for any change of water taste or quality".
Subsequently one customer reported a change in "water quality and taste", and investigations found the property had an "incorrect connection between the recycled water main and the drinking water supply", she said.
All drinking water meters in the area were then tested, revealing another two properties had incorrect water connections.
The supply comes from the Melbourne Water Western Treatment Plant in Werribee, which uses large ponds to treat sewage to either a Class A or Class C recycled-water standard.
The spokeswoman said Class A was the "highest class" of recycled water, and was treated so that it could be used in the irrigation of vegetables, for watering gardens and washing clothes.
She said the water connected to homes in Melbourne's west was referred to as a "shandy", a mixture of clean drinking water and Class A recycled.
"Recycled water is produced at Melbourne Water's Western Treatment Plant where it undergoes a very high level of treatment to become Class A recycled water," she said.
"The recycled water then undergoes further processing to reduce the salt level at our West Werribee facility which includes mixing approximately 50 per cent recycled water with 50 per cent drinking water."
The spokeswoman said the likelihood of those who drank the water falling ill was "low", however City West Water is working with the Department of Health and Human Services to assess any potential impact to health from drinking this water.
"This information will be made available once complete," she said.
The situation has since been rectified and taps at the three affected houses are no longer runningh recycled water.
City West Water has appointed an independent expert to undertake a "comprehensive investigation" to determine the cause, which will be completed by the end of the week, the spokeswoman said.
Anyone concerned about their recycled water connection can contact City West Water on 132 642.