The owners have not yet been found because it was not microchipped, he said.
"He had to be put down but we're still quite keen to find out who the owners are.
"This is obviously not the ending we wanted and we're sorry if we got anyone's hopes up.
"We had a lot of media interest in the story of this cat, and thanks to everyone for your help, but despite our best efforts this is the sad outcome."
The neutered cat was a short-haired male, about 12 to 15 years old.
SPCA spokeswoman Andrea Midgen said the cat's injuries were too severe for it to be treated.
"The cat had multiple fractures to its pelvis so it was in a lot of pain and it wasn't something that was going to be repairable.
"Unfortunately the best thing we could do for the animal was to put it down."
Call Mr Lawes on (09) 839 0600 if you think you are the owner.