The latest survey released on Friday estimated there were 18,814 horses in the park in November 2022, compared to 14,380 in late 2020.
That is despite a government commitment to reduce the horse population to 3000 by June 2027 using a range of control measures implemented by the NSW National Parks and Wildlife Service (NPWS).
A NPWS representative attributed the latest increase in horse numbers to two "good seasons", following years of bushfires and drought which they said impacted the previous survey.
The NPWS "removed" 859 horses from the park in the 10 months after the latest Wild Horse Management Plan was adopted by the government in November 2021.
A NPWS representative declined to clarify what proportion of the horses were culled and how many were removed through other control methods such as rehoming.
Several native species at risk of extinction rely on habitat within the national park which is dramatically degraded by the horses.
NSW Environment Minister James Griffin said the government was delivering wild horse control in line with the requirements of the plan.
Ecology and conservation expert Don Driscoll from Deakin University blamed the government for ignoring scientific advice for years and creating a much larger problem.
"The Australian Academy of Science has repeatedly urged the NSW government to do a better job of controlling the feral horses," Professor Driscoll said on Friday.
"They have repeatedly ignored that scientific advice and now they've got almost 19,000 horses to deal with.
"If they're going to get on top of this problem, they really have to rapidly increase the numbers of horses they're taking out of that park because the natural rate of increase means what they've done so far hasn't even been a drop in the ocean."
Greens MP Sue Higginson said the NSW Liberal party was being held captive by the Nationals' opposition to brumby culls.
She called for even greater reduction targets.
"The government has ignored the science and played politics with this national treasure," Ms Higginson said.
"Even the government's target of 3000 by 2027 is too many when we are talking about a national park."
Animal Liberation regional campaigns manager Lisa Ryan urged the government to consider other options fro dealing with the brumbies including sterilisation programs or creating specific areas for the horses.
"We understand there are two sides here but we still are very strongly opposed to cruel and inhumane culling methods when there are other options," Ms Ryan said.
"Whether people like it or not, the horses are part of Kosciuszko's history."
Invasive Species Council advocacy manager Jack Gough said culling by highly trained professionals using protocols reviewed by the RSPCA was the only viable way to reduce feral horse numbers.
"While we understand that, after years of inaction, initial control efforts would be slow to ramp up and appreciate that there was a small increase in the rate of removal in the second half of 2022, the rate is nowhere near what is needed to start to reduce numbers and protect this unique landscape," he said.
"The future of unique wildlife such as the corroboree frog and sensitive alpine wildflowers and the headwaters of the Murray and Murrumbidgee rivers are at stake if horse numbers are not brought down rapidly."
The latest survey also recorded a doubling of the deer population in Kosciuszko National Park, despite NPWS removing more than 6800 of the species in the two years to November 2022.
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