Ms Jones said the injuries are not recent and have become infected.
"He has had one anaesthetic so far to remove the arrow and clean his wounds," she said. "Over the weekend he has been on IV fluids, triple combination IV antibiotics and a medication for pain. We have concerns about his leg and are currently seeking specialist advice with a view to providing the most appropriate treatment to save the leg."
Ms Jones said Apollo may need specialist surgery to save his leg, and it may be safer to leave the pellets behind his elbow and in his chest as long as infection is prevented.
SPCA Auckland chief executive Christine Kalin described the case as on the "upper end of severity".
"I don't see how someone can do that to an animal," Ms Kalin said. "As the frontline organisation for animal welfare, we do often have cases where horrific things have happened. In terms of severity, this is on the upper end of severity.
"One of our warrant inspectors is currently investigating the incident as this is a clear breach of the Animal Welfare Act and we would like to be able to identify the person that committed this dreadful and horrific act to ensure that they are held accountable."
It is believed the attacks on the cat happened about three days before he was found.
Ms Kalin said it is not known who is the owner of Apollo, but if someone recognises the cat he will be returned once he has fully recovered. Otherwise the SPCA is keen to re-home him.
The president of the Massey Archery Club, Mary Thomas, said the club was assisting with the SPCA's investigation.
Ms Thomas said the club is trying to identify where the arrow came from and members have been asked whether they saw someone on the grounds on the day.
"We've seen the arrow and photos of the cat, and we believe that arrow was not shot with the bow that it was made for. What we think has happened is that someone has overshot a target and not been able to find their bow and that someone else has picked it up mucked around with it."
She said it is a possibility someone from the club may be responsible, although the club operates on 13 acres of bush and often has trespassers.
"We do get people in there that are not members, just mucking around and that. We've got danger signs, private property signs up all over the place but it doesn't stop them."
The club has made a donation towards the recovery of the cat, although Ms Thomas believes none of club's members are responsible.
Ms Kalin asked anyone with information that may assist their investigation to contact SPCA senior inspector Kevin Plowright on 027 278 0907.
- Herald Online