A permit to kill up to 200 black swans in Tasmania's Central Highlands was approved after the native bird rendered the area unsuitable as a habitat for trout.
A property protection permit was issued earlier this year by the state's Department of Natural Resources and Environment (NRE Tas), with around 70 birds culled so far in the London Lakes area.
Environmentalist Bob Brown from the Bob Brown Foundation said it was "needless slaughter".
He likened the situation to the Tasmanian tiger, which was hunted to extinction to protect livestock.
"Here we've got another case of native wildlife being destroyed to protect an introduced species," Mr Brown said.
"There's going to be popular revolution at the thought of breeding swans and their cygnets being killed because they are disturbing an introduced species."
An NRE Tas spokesperson said the permit was granted to disperse a large concentration of several thousand swans from the water body.
"They were fouling thereby rendering it unsuitable as a trout habitat," the spokesperson said.
"Recent waterbird surveys confirm that black swans continue to be widespread and locally abundant through central Tasmania and that taking up to 200 individuals was not considered a threat to their regional population."
Mr Brown said it was "appalling" and had the potential to damage Tasmania's image.
"Both things can co-exist, they have done so for decades, there is no need for this slaughter," he said.
Trout were first introduced to Tasmania from England in 1864, with the state now known around the world as a premier destination for anglers.
'We just don't value our wildlife'
More than 8,000 swans were culled in Tasmania between 2015 and 2018, while thousands more have been killed in the years since.
Dr Eric Woehler from BirdLife Tasmania said the cull at London Lakes was another example of the state government prioritising an introduced species.
"Another permit to cull black swans is just a sad indictment of the fact that we just don't value our wildlife," he said.
"It shows a complete lack of respect for our native wildlife. It is just unacceptable.
"It's hard to reconcile our clean and green image we project to the world with this slaughter."
Permit system 'broken'
Tasmanian Greens MP Rosalie Woodruff said the cull was "beyond belief".
"Minister for Primary Industries Jo Palmer needs to immediately intervene and stop this gross destruction of protected native wildlife," she said.
"It is past time to end the prioritising of economic interests over the survival of Tasmania's native wildlife."
She said the department's property protection permit system was "broken" and needed fixing.
"It views protected native wildlife as expendable and necessary collateral damage to commercial interests," she said.
A cull of wallabies at a northern Tasmanian golf course due to "excessive droppings" and "destruction" of the course's greens was abandoned last year after community outrage.
In 2019, the Launceston Golf Club angered animal welfare advocates and some local residents when they received approval to cull native species.
The club also decided not to go ahead with the planned cull.
