Wildlife Protection Association spokesman Pat O'Brien said Tuesday his group was seeking legal advice about a possible court injunction to stop the slaughter at a former navy station in Canberra.
If that fails, activists are prepared to try to get in the way of officials taking part in the operation, he said.
"People are threatening to climb the fence to disrupt any killing that's taking place," O'Brien said. "It's going to be pretty nasty."
The Defense Department has approval from the Australian Capital Territory government to cull the kangaroos on the former base because they have become too numerous and are at risk of starvation.
Military officials have erected temporary fences on the site in preparation for the cull, and spokesman Brig. Andrew Nikolic said a contractor could begin within weeks.
A government report recently recommended killing the kangaroos by first shooting them with a tranquilizer and then euthanizing them by injection.
The plan is a scaled-down version of an earlier proposal to eradicate about half of the more than 6,000 kangaroos at two military sites in the capital because their numbers had grown too large and they were eating themselves out of a habitat.
The initial proposal was to allow the Defense Department to shoot 2,800 kangaroos at one of its properties and use tranquilizer darts to kill the 400 on the naval property, the Belconnen Naval Transmission Station, where firing bullets is regarded as too dangerous to the public.
The plan was put on hold after activists launched a campaign claiming the cull was inhumane.
More than 60 species of kangaroo exist throughout Australia, and while a few species are endangered, others have thrived on Outback land that has been cultivated for grazing. They now exist in huge groups, known as mobs.
Federal environment protection laws allows for harvesting of wild kangaroos, and they are killed by licensed hunters under a quota system. The meat is used for human consumption and pet food, and the pelts for soft toys and other items.