It's thought 16 cats found dead in bushland southwest of Sydney were deliberately drowned.

An animal rescue group called police to an area behind an industrial estate at Silverdale after making the discovery on Thursday.

The bodies of the animals were in various stages of decomposition and a post-mortem conducted on some of the remains indicated they were drowned.

Police were told the cats were living in bushland and were not microchipped.

The RSPCA is working with police to try to find the culprit.

Meanwhile, a young couple is distraught after finding their pet cat shot dead with an arrow further south at Tahmoor.

The body of three-year-old Ra was discovered in their backyard late on Thursday night.

The cat's distraught owners, a couple in their 20s, have told police they do not know who may be responsible.

Police have described the crime as disturbing.

"While some people may not consider this to be a major crime it is an abhorrent offence that touches the hearts of many members of our community and is of great concern to NSW Police," Detective Inspector Jayne Doherty said on Friday.

Officers weren't aware of any complaints about Ra, while the arrow used in the attack had been recovered and sent for forensic testing.

"From our early inquiries, it appears this pet was clearly targeted by someone armed with a bow and arrow in a public place," Det Insp Doherty said.

"This was not a feral cat, it was a couple's much-loved pet with a collar and tag."