The Defense Department wrote to animal welfare groups Thursday to cancel a meeting planned for Friday to discuss the cull, saying the talks were "probably premature" and that other options were being considered.
"Defense is still considering the available options for kangaroo management at both sites," the department's regional manager Larry Robbins wrote to the groups, without elaborating on those options.
Defense sparked a furor when it applied early this month for local government permits to hire professional shooters to cull the kangaroos. It said the animals are in plague numbers at two sites the department owns on Canberra's outskirts, and threaten to eat themselves and other animals into starvation.
O'Brien suspected the department was backing down because of hundreds of complaints directed at Defense Minister Brendan Nelson in an election year since the cull plan was revealed earlier this month.
"I'm sure there's been a lot of political flak flying around the place over this, so they're rethinking their options," O'Brien said, adding it was too early to claim a complete victory.
Leading animal ethicist Peter Singer, professor of bioethics at Princeton University, said the cull was "not ethically justifiable" because the kangaroos were not starving or in great distress.
"Kangaroos are sentient beings who can enjoy their lives," Singer told AP. "As long as they can do so, it is better to let them continue to live."
Veterinarian Hugh Wirth, former president of the World Society for the Protection of Animals and of the Australian equivalent, Royal Society for the Prevention of Cruelty to Animals, condemned the proposed shooting of 3,200 of an estimated 6,500 kangaroos as a slaughter.
But Wirth said culling to prevent over population should be a continuous process of killing the old, infirm and drought-starved.
"They should have been controlling the population long ago," Wirth said of the authorities.
"They can't fix months and months, in fact, years of neglect in one all mighty kill - that's a slaughter, not a cull," he added.
"Of course it triggers public outrage, and justifiably so," he said.
Canberra's local administration, the Australian Capital Territory government, has yet to approve licenses for the cull.
Defense said in a statement released later Thursday that the department's "primary concern is for the welfare of the kangaroos, the sustainability of the kangaroo populations and of the total environment under Defense control," including flora and fauna.
The department has previously said the kangaroos were not only threatening their own survival, they were destroying the habitat of threatened species including the Grassland Earless Dragon, Striped Legless Lizard and Golden Sun Moth.