Melbourne's Lort Smith Animal Hospital is making a desperate plea for pet owners to pay their bills.

The hospital helps tens of thousands of animals every year, but now it needs a hand to recover the $4 million it is owed.

Pet owners who cannot afford a trip to the vet can seek help from the hospital at a reduced cost, but many have failed to pay up.

The hospital is now using rescued dog Macca to tug on the heart strings of pet owners to do the honest thing and settle their outstanding bills.

The hospital's CEO Liz Walker told Seven News reporter Emily Angwin: "Please start to talk to us and communicate with us. If you owe us something then please talk to us."

More than 30,000 animals are treated at the hospital every year but if people actually paid their bills, they could help more.