Harmful blue-green algae has been seen in the Werribee River for the second summer in a row.
Melbourne Water advised locals that they and their pets should not go in the river, downstream of the Maltby Bypass, because of the algae.
The algae forms blooms during hot weather, in periods of drought or low flow, or when there is an accumulation of nutrients in waterways or water bodies.
Waterways manager Cameron Howie said Melbourne Water would monitor the problem through regular water sampling and had erected signs to warn people of the potential danger of coming into contact with the water.
“It’s important to ensure the water does not come into contact with skin or eyes,” Mr Howie said.
Melbourne Water released about 160 million litres of water into the Werribee River from the Melton Reservoir at the weekend to improve water quality in the lower reaches and to provide relief for aquatic plants and animals after a hot, dry start to summer.
The algae comes three months after a Wyndham council report highlighted the poor state of the river, declining platypus numbers, water quality ranging from fair in the upper reaches to very poor in the lower reaches, and outbreaks of Azolla weed and algae.
A survey also recorded a staggering 50,000 items of litter found in the river.