By Phil Mercer
BBC News, Sydney
The kangaroos are being held in a disused army base
Animal welfare groups in Australia are taking legal action to try to stop the killing of up to 6,000 kangaroos on a disused military base near Canberra.
Officials have said that there is an overpopulation of kangaroos, and that they are damaging the environment.
But campaigners say the killings are a "brutal and unsubstantiated slaughter".
Earlier this week several protesters broke into the Majura army base near Canberra, in an unsuccessful attempt to stop the beginning of the cull.
They've promised more direct action should the legal challenge fail, and they're applying to a civil tribunal to have the cull stopped.
Nikki Sutterby, from the Australian Society for Kangaroos, says the military has provided no compelling reason why so many animals need to be shot.
"They have commenced a slaughter and they are alleged to have killed around 2,000 kangaroos within about four days," she told the BBC.
"This is being done without any scientific proof or research to back it up."
She said the animals would have suffered badly, claiming that joeys would have been separated from their mothers, and family groups would have been destroyed.
Australia's Defence Department claims the marsupials were overgrazing native grasslands, and that endangered plants and insects were being threatened.
Officials have also stressed that the animals are being killed humanely.