A Canbarra man went on a kangaroo shooting rampage inside a national park, leaving some animals to die slow and painful deaths, police and parks officials say.

Police have charged the man with illegally killing more than 20 kangaroos in the Namadgi National Park near Canberra, Australia.

The man, 30, is alleged to have shot the kangaroos over a three-month period from November 2006.

Police have seized the rifle allegedly used in the killings and will claim the man killed 18 kangaroos and left another two alive but so badly injured they had to be put down by rangers.

Ballistics specialists are analysing the bullets and the weapon, police said.

Australian Capital Territory's (ACT) parks director Russell Watkinson declared the killings "abhorrent".

"Some of the kangaroos were shot in the hip or the tail and left to die a slow and painful death – it was a cruel and senseless act," Watkinson said. "In addition, the shootings took place in a national park which is open to the public, potentially creating a very dangerous situation."

ACT's Senior Constable Brian McAlonan said police and rangers were working closely to combat rural crime.

"Offences such as killing native animals carry a maximum penalty of one year imprisonment and are taken very seriously by both police and the rangers who patrol the national parks day and night," Constable McAlonan said.

The suspect will face court on August 10, charged with killing a native animal, discharging a firearm in a public place and failure to keep his firearm registration up to date.