Three kangaroos, including a foetal joey, have been killed, mutilated and dumped in suburban Melbourne.

Lysterfield South resident Robert Bishop made the grisly discovery near his home, which borders the Churchill National Park, after hearing a group of men attacking the animals early on Wednesday.

"It's so horrific - I don't understand why anyone would do such a thing," Mr Bishop told AAP on Monday.

He had ventured out after a loud bang, expecting to find hoons, instead seeing six young men hacking at a twitching kangaroo with a "big blade".

Feeling unable to take on the men, Mr Bishop yelled at them before retreating inside and calling police.

Once the men had left, Mr Bishop took photos of the disembowelled animals, including a decapitated foetal joey, which he has passed on to police and the RSPCA.

A Victoria Police spokeswoman said they were investigating the matter.