Greens Minister Shane Rattenbury is under pressure from his own party to stop the kangaroo cull, under which 1606 kangaroos on eight Canberra nature reserves are to be shot over the next six weeks.
NSW Greens convenor Hall Greenland has written to Mr Rattenbury, who is Territories Minister in the ACT Labor government, asking him to call off the cull, which has sparked protests from animal welfare groups since it was announced last week.
The cull was to begin on Wednesday, May 14, with protesters vowing to walk the reserves at night and stand in front of the shooters, but was temporarily halted after Animal Liberation ACT launched a challenge in the ACT Civil and Administrative Tribunal.
A hearing is set down for Tuesday, May 20.
Animal Liberation made a similar attempt last year but the tribunal allowed last year's shoot to go ahead, at reduced numbers.
Mr Greenland said the mass cull should be cancelled on the grounds of animal cruelty and because there was no evidence to support the government's claim that the kangaroos were threatening the survival of other animals and plants.
The party's animal welfare group had looked closely at the issue and was concerned about the lack of data to back the government's claims.
If kangaroos were damaging the reserves, then non-lethal options should be considered first, the group said.
"We are concerned that, once again, the ACT government has signed off on a brutal cull of kangaroos without evaluating the real impact of the population and more humane methods of control," Mr Greenland said.
"The frequency of the cull also suggests that the ACT kangaroo management plan is not delivering the desired outcome."
But Mr Rattenbury is pushing ahead, saying that while some in the ACT Greens shared the NSW view, others agreed with him that the cull was a "necessity from an ecological perspective".
"This is an issue where there always been different opinions within the Greens," he said.
"Some see the necessity from an ecological perspective of a cull and others oppose it on animal welfare grounds, so it is clearly a difficult issue for our party."
Mr Rattenbury said he had looked closely at the issue and believed that the cull was necessary.
He rejected the assertions of the animal welfare groups that the science was nowhere near as clear-cut as the government claimed.
"There's a difference between saying there's no science and disagreeing with the science that is available," he said.
"People will debate the science, and that’s a healthy thing, but the best science available to the government points to the fact that overgrazing is an issue in woodlands and grasslands."
All previous Greens assembly members, including Kerrie Tucker and the four Greens in the last assembly, had shared his viewpoint, Mr Rattenbury said.
But he, along with all the Greens, would much prefer a non-lethal solution, which was why he was working with Alphadog to trial a drug for fertility control of kangaroos.
Much work is yet to be done on that trial, with Alphadog director Marcus Fillinger saying he was upset that the government was going ahead with a cull alongside the trial.
Mr Rattenbury said he, like Mr Fillinger, wanted the trial to be scientifically rigorous and measurable.
The annual cull began in 2009, when 494 kangaroos were killed. Last year, 1149 kangaroos were killed, along with another 355 "pouch young".
Mr Greenland congratulated Animal Liberation on its legal challenge.