SPCA executive director Bob Kerridge said clubbing was a cruel way to kill an animal and questioned the veterinary report.
"How does he know he killed it in one blow? That report might say that but it is a grey area," Kerridge said.
Under the Animal Welfare Act it is only an offence to kill an animal if it causes unreasonable or unnecessary pain or distress.
Taramoeroa said this week he regretted killing a stray cat to protect his own moggie but claimed it wasn't cruelty - and the law has let him off, too.
The self-employed truck driver said he was an animal lover and his cats were like "my children".
"This was a recurring problem and an opportunity presented itself to deal to the problem in a manner that wasn't villain ... I regret killing any animal but when it comes to defending my own ..."
Taramoeroa said several stray cats lived under a public library nearby and often came on to his property to fight his pets.