Latest estimates reveal there are at least 6476, and as many as 16,411, of the feral horses roaming NSW's Kosciuszko National Park, near the Victorian border.
The 2025 figures released on Friday showed a significant rebound from the year before, which recorded a lower estimate of 2131 and upper number of 5639 horses.
It comes after aerial culling efforts were put on hold in March 2025, following a successful reduction in numbers, amid claims the practice was cruel and resulted in some brumbies being shot and left to die.
Damage to the park's fragile ecosystem by the wild horses remains widespread and won't cease until the animals are essentially removed, some culling advocates say.
Under its controversial management plan, the NSW government is working toward a legislated target of less than 3000 brumbies across the park by mid-2027.
In 2022, when culling began, the number of horses in the park was as high as 23,535, with trapping, rehoming and ground shooting reducing numbers by only a few thousand.
The introduction of aerial culling in late 2023 resulted in a drastic reduction, with more than 6000 animals shot from helicopters.
Aerial shooting will resume in June, while rehoming of horses from retention areas begins on Friday.
The National Parks and Wildlife Service will also assess plans for a fertility control trial based on global evidence.
Invasive Species Council chief executive Jack Gough said population boom cycles would continue while large populations were allowed to persist.
'It's simple biology – feral animals do not stop breeding because control efforts are paused for a year," he said.
"The consequence of that will be even more horses will need to be culled."
Mr Gough argued rehoming and fertility control would not meaningfully reduce horse numbers.
"We are dealing with thousands of feral horses across rugged alpine country," he said.
The council is also calling for the scrapping of retention zones which apply to 32 per cent of the park, introduced under the Kosciuszko Wild Horse Heritage Act.
NSW Environment Minister Penny Sharpe said the latest survey showed much more work was needed to control the population.
"No one wants to have to kill horses. But there are still too many in Kosciuszko National Park," she said.
"We will use the best available science and take a cautious, evidence-based approach to reach the required population target, in order to protect native vegetation, animals, waterways and cultural values."
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