Two kangaroos were shot by arrows in the past two weeks, in what park rangers have described as ''distressing'' attacks on Mount Ainslie.
One of the kangaroos was found already dead, and the other had to be put down to end its suffering.
The maximum penalties possible would be two years in prison and up to $22,000 of fines.
But the RSPCA said those punishments were not enough, and Animal Liberation ACT said that ACT courts rarely passed down maximum penalties for animal cruelty offences.
RSPCA ACT chief executive Michael Linke said ACT laws surrounding animal cruelty were relatively weak compared to other Australian jurisdictions.
He urged the ACT government to look at toughening penalties contained in the Nature Conservation Act or the Animal Welfare Act.
''We're a long way behind other states, $22,000 and two years seems a lot, but when you compare that to states where penalties can get up to $75,000 and five years in jail in NSW and Queensland and places like that,'' Mr Linke said.
''To have the government look at these fines and perhaps give magistrates greater grounds and greater scopes to impose heftier fines would be a positive move.''
Animal Liberation ACT manager Bernie Brennan said the attacks were, in part, evidence of the demonisation of kangaroos as unwanted pests.
''It saddens me, but it doesn't surprise me,'' Mr Brennan said.
''The kangaroo has been sold and demonised as a pest, and now that they're considered as a pest, there's just no compassion for them at all.''
ACT Parks and Conservation ranger David Whitfield was part of a team that found the second kangaroo, which had an arrow pierced through its back.
Mr Whitfield and other rangers have pleaded for the help of the public, but they admit it was unlikely the culprits would be caught.
''It's possible that it could happen again, but hopefully through ... increased presence of both rangers and the police, the person would become very aware that what they've done is wrong, and let's hope their conscience sorts that out for them, and they don't do it again,'' he said.
Police and park rangers have stepped up their monitoring of the area in an attempt to catch the culprits.
Anyone with information should contact Crime Stoppers on 1800333000.
By CHRISTOPHER KNAUS, Canberra Times