UPDATE: The City of Greater Bendigo says it is aware of bird deaths at Lake Weeroona.
Parks and Natural Reserves manager Simon Harrison said it was too early to say what had caused the deaths.
“The City of Greater Bendigo is aware of the deaths of a small number of birds at Lake Weeroona," he said.
"The cause of death is unknown at this stage, however it is not uncommon for birds to pass away at the Lake and (council) staff actively remove any birds that have done so.
"The (council) tests urban lakes for Blue Green Algae only.
"December results show there were low levels of Blue Green Algae present and January testing results are yet to be returned."
Mr Harrison said botulism could be present in the lake.
"Botulism occurs naturally in soil and the environment, so it may be present in low levels," he said.
"For the past 14 days, the (council) has also been topping up the Lake with class A recycled water, the highest quality of water available to top up lakes and reserves.
"This process aerates the water and improves the overall quality, helping to reduce the temperature so as there are no algal bloom outbreaks.”
EARLIER: Wildlife Rescue and Information Network officers tried to rescue five birds from Lake Weeroona over the weekend after it was found a toxin in the water was making them sick.
Four of the birds had to be put down and one other is clinging to life.
WRIN officer Lynne Waller said the lake was toxic at the moment.
"In my opinion it should be off limits," she said.
"There are dead and dying birds there and people should not be fishing, letting their dogs in it or doing anything with the water."
Ms Waller said a Moscovy Drake, two Black Pacific ducks, a hybrid drake and a juvenile dusky moorhen were taken out of the lake.
"Two were already dead and bought into me with the (hybrid) drake," she said.
"The moorhen was euthanised last night and the Moscovy Drake was euthanised on Saturday night.
"(The toxicity) is serious and something needs to be done fast.
"We can't be losing birds like this."
Ms Waller said affected birds would come to the edge of the water and flop on one side or swim around with their heads underwater, which put them at risk of drowning.
Ms Waller believes the birds have died of botulism, an illness that can be fatal to humans as well as animals.
Botulinum toxin, which is the cause of botulism, is used in forms Botox and cosmetic surgery.
Comment is being sought from the City of Greater Bendigo, the EPA, and the Department of Environment and Primary Industries.