A local wildlife shelter alerted Victorian wildlife officials after finding the eastern grey kangaroos in rural paddocks about 120 kilometers (75 miles) north of Melbourne, officials said.
"At least three of the kangaroos were found alive but were subsequently euthanized due to the severity of their injuries," said the state's Conservation Regulator.
"Two kangaroo joeys were recovered and are being cared for at a wildlife shelter," it said in a statement.
The marsupials had suffered fatal gunshot wounds and "traumatic injuries consistent with being rammed by a vehicle", the regulator said.
Used shotgun cartridges were found in the area.
Though kangaroos are protected in Australia, the most common species are not endangered.
This means they can be shot and killed in most jurisdictions, but government permission is needed.
Kangaroos, whose numbers fluctuate between 30 million and 60 million nationally, are frequently culled to keep the population in check.
The animals have a "boom and bust" population cycle -- when fodder is plentiful on the back of a good wet season, their numbers can balloon by tens of millions.
Each year, as many as five million kangaroos are also shot as part of a homegrown industry that harvests their carcasses for meat, pet food and leather.