The Environment Protection Agency (EPA) says there is nothing it can do to reduce the odour from dead and dying fish at Lake Boga, near Swan Hill in north-west Victoria.
The Swan Hill Rural City Council says it is waiting for advice from the EPA about managing the odour.
But the EPA's acting chairman Bruce Dawson says the only way to remove the odour is to remove the fish, but that is not possible because of their state of decay and because deep mud around the lake is preventing access.
"The sheer scale of the incident up there at the moment and the number of fish that have died means that there isn't any option other than some attempt to remove the fish," he said.
"Unfortunately that hasn't been successful and under the current conditions is unlikely to be successful.
"If the conditions were to change so that access to the water was possible - if the mud was to dry out and harden to enable vehicles to access it, that sort of thing - unfortunately that's probably not going to happen during the time the fish decompose, but we are evaluating that each day."