Australia's environment minister defended plans Wednesday to slaughter 400 kangaroos in the national capital because they are overeating the region's grass, but protesters vowed to prevent the cull.

The government's 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 Canberra because their numbers had grown too large and they were grazing themselves out of a habitat.

However, the proposed killing of Australia's beloved national symbol has triggered international protests from animal protection activists.

Wildlife Protection Association spokesman Pat O'Brien said Wednesday he received legal advice that a court challenge to the cull was unlikely to succeed, so activists planned to block the site "to anybody that wants to go in and kill them," he told Australian Broadcasting Corp. radio.

Environment Minister Peter Garrett defended the plan and dismissed calls for the federal government to use its powers to stop the cull.

"Australians care a great deal about their environment and about their wildlife," Garrett told reporters. "But when there are significant imbalances and the possibilities that you'll have conditions which don't benefit the environment and wildlife in the long-term, then programs like this — humanely and properly administered — are sometimes necessary."

Authorities argue kangaroo overpopulation has caused overgrazing of rare native grasslands that also threatens species such as the grassland earless dragon, striped legless lizard, and golden sun moth.

Garrett, a former rock star who is now a leading figure in the international campaign against Japanese whaling in the Antarctic Ocean, rejected comparisons between slaughtering whales and Australia's iconic fauna.

"Our views on whaling are clear and we will continue to resolutely oppose the killing of whales in the Southern Ocean," said Garrett, former frontman of the rock band Midnight Oil.

The Defense Department initially planned to cull the kangaroos with guns until police raised public safety concerns in the suburban environment. The plan now is to shoot the kangaroos with tranquilizer darts before administering lethal injections. No timetable for the cull has been announced.

Defense Minister Joel Fitzgibbon and the head of Australia's defense force, Air Chief Marshal Angus Houston, declined to comment Wednesday.

Federal environment protection laws allow for harvesting of millions of wild kangaroos, which 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 quality leather.

The vegetarian British animal welfare organization Viva! this week launched an international petition to pressure Canberra's local government to allow the kangaroos to be relocated rather than killed.