Forest & Bird says the Department of Conservation's tahr control plan breaches the National Parks Act, and does more for wealthy overseas heli-hunters than protecting the environment.
DOC has said it will continue working with the hunting community to reduce tahr on conservation land.
"DOC's Tahr Control Operational Plan sets to remove 10,000 tahr by the end of August 2019 and we are committed to working with the recreational and commercial hunting sector to achieve this," DOC's Director Community Engagement, Dr Ben Reddiex, said in a statement.
"Initial control efforts will focus on tahr exclusion zones and in associated buffer areas."
Approximately $1 million had been allocated for Himalyan tahr control, and further research into tahr abundance and its impacts on the environment until August this year.
"Urgent action is needed. It is important we stop the population migrating further than the current feral range.
"There is no plan to eradicate tahr however, we need to ensure that New Zealand alpine ecosystems are protected from the growing tahr population," said Reddiex.
However, Forest & Bird said the Department's plans to leave bull tahr behind and simply tell hunters where they are was a problem in areas where zero density was required, such as Aoraki / Mount Cook and Westland Tai Poutini National Parks.
"We've got vast herds of tahr up there doing enormous damage to alpine plants," said Forest & Bird lawyer Peter Anderson.
"This is urgent. In National Parks, it's not lawful to leave behind large numbers of trophy animals which hunters may or may not kill."
The 1993 Himalayan Tahr Control Plan set the total allowed tahr population in the central South Island mountains at 10,000 animals.
Work by the Department of Conservation this past summer gave increased confidence that the population is instead around 35,000.
Within Management Unit 4 - which includes Aoraki / Mount Cook and Westland Tai Poutini National Parks as well as part of the Liebig Range - the allowed tahr population is set at 500 animals outside of the National Parks.
Instead the current population in the management unit is estimated as 15 times larger, at 7666 animals.
"Within those National Parks, to aim for anything less than zero density is actually illegal," said Anderson.
"The reason that we're in this situation is the Department of Conservation abandoned tahr control, and the population ballooned."
"Given that the animals are breeding, to get back down to a population of 10,000, we need to give it everything we've got."