Activists have set up camp outside Puckapunyal army base in a bid to stop the 'slaughter' of 6000 kangaroos.
Australian Society for Kangaroos president Nikki Sutterby said the group hurried to the base on Monday after receiving a tip that a cull had begun.
She said ASK planned to stay at the site until the Department of Defence stopped the cull.
''Kangaroos do no damage to the environment and do not need to be killed to restrict their numbers. It is Australia's dirty little secret what we do to our kangaroos,'' she said.
A Department of Defence spokesman confirmed a cull had been ordered and that up to 6000 eastern grey kangaroos would be killed by March 2014.
The spokesman estimated the base had an eastern grey population of about 40,000 and said the cull was required to manage overpopulation.
A spokesman for the Department of Environment and Primary Industries confirmed a permit to manage kangaroos at Puckapunyal was issued.
Ms Sutterby said ASK did not believe there was a need to cull the kangaroos and that Defence was 'doing it as a precaution'.
''But you don't kill healthy kangaroos and their joeys just in case they become starving in the future,'' Ms Sutterby said.
''Kangaroos are adaptive and if left to their own devices, they will find equilibrium.''
The group believed 'shooters' were not following codes of practice and were leaving joeys to die and kangaroos were not being shot properly.
Defence said the cull was being undertaken by specialist contractors in compliance with the national code of practice for the humane shooting of kangaroos and wallabies.
The spokesman said this included 'the humane destruction of joeys'.
''Defence is committed to sustainable and best-practice environmental management and has an obligation to ensure the protection of endangered ecological communities and to manage overabundant species,'' the spokesman said.
At the height of drought in 2002, the army base became a killing field when about 30,000 starving kangaroos were killed.
The Department of Defence estimated at the time that the kangaroo population before the cull was in excess of 80,000.
Ms Sutterby said ASK believed 'several thousand' kangaroos had been killed each year since.
''Research conducted recently in South Australia showed that if they were left alone, kangaroo numbers would increase about 10 per cent each year and once they reached equilibrium, the population will stabilise,'' she said.
''There are significantly less kangaroos there than ever before.''
The loss of life has angered the RSPCA and has prompted it to call on the DEPI to review the Puckapunyal kangaroo management plan.
''Crisis points such as this are likely to be the result of poor long-term planning and ineffective management,'' chief executive Maria Mercurio said.
''The obvious reason for any large-scale cull is either an absence of a long-term management plan or, if a plan is in place, its inadequacy.''
She said if methods of long-term planning, prevention and management were carefully considered and implemented, then crisis points such as this would be avoided.
Ms Sutterby said lowering the number of kangaroos at the base would result in wildfire risk with 'inadequate' native wildlife present to maintain the natural habitat.
But the Department of Defence said it managed its fire risk and kangaroos did not perform a role that mitigated fire risk at the base.
Ms Sutterby said Australians were 'kept in the dark about kangaroos' and many were led to believe that culling was unavoidable.
''But killing them is extremely cruel and unnecessary and most of it is about profit for the kangaroo industry,'' she said.
Sydney Morning Herald