SPCA Auckland vet manager Shalsee Vigeant confirmed the cat, whose sex is yet to be determined, died by drowning.
"It is a very sad and inhumane way for any animal to die," Dr Vigeant said.
SPCA Auckland chief executive Christine Kalin described it as "an appalling act of animal cruelty".
"We need to find the culprit quickly and seek justice for this innocent animal," Ms Kalin said. "This cat may have been someone's beloved pet or it may have been a stray but either way the person who killed it has committed a crime.
"We are appealing to members of the public to call us with any information that might lead to the arrest of the offender. If you have seen someone throwing something that looked like this trap into the harbour, or have any other information, please call us on 09 256 7300."
- nzherald.co.nz