Four cats have died and a fifth needed a blood transfusion after poisonings in Papamoa.
Another cat died after it was shot twice with an air rifle.
Papamoa cat Tigger died on Thursday night after it was shot twice with an air rifle earlier this week. Owner Leslie Welch said her cat had "deteriorated" throughout the week and died as a result of his injuries.
She has reported the matter to police.
A three-year-old cat named Poxy died on Tuesday after it was found convulsing in the garden.
Poxy's owner Andrew Henderson said his family was "absolutely gutted". Melissa Henderson said their three children were "heartbroken".
Poxy was the third pet cat the Hendersons had buried in two weeks - Pretty Girl died on April 21 and Wiglet was euthanised on April 24. All three cats showed signs of being poisoned and the Hendersons now feared a serial cat poisoner was on the loose in Papamoa.
Tauranga SPCA animal welfare inspector Jason Blair said the circumstances seemed suspicious, particularly as three cats from one family had been affected.
Owner and senior vet at Bayfair and Papamoa Vets, Paul Greaves, said the Hendersons' three cats appeared to have ingested three different poisons - slug poison, rat poison and anti-freeze. He said Pretty Girl's convulsions - first detected by the Hendersons' three-year-old son - were a sign of metaldehyde, slug poison, which was fatal if ingested.
"Dogs are more likely to eat slug bait than cats so this sounds malicious." The second cat, Wiglet, appeared to have eaten rat bait, which was an anti-coagulant and prevented blood from clotting.
When the Henderson's took their third cat to Papamoa Vet, Mr Greaves said the cat was "almost dead".
"It was lethargic, depressed and severely dehydrated. It also had halitosis or uric breath which is an indicator of kidney failure, and this is secondary to oxalate toxicity which is likely to be caused by anti-freeze."
On Monday, another Papamoa family's cat died. Veterinary tests confirmed it had ingested anti-freeze and suffered chronic renal failure.
Mr Greaves said the cat was uncoordinated and unable to stand or walk. When it was picked up, its front legs went into spasms.
"It's what we call uraemic fits and it makes the front legs all rigid ... a result of kidney failure," Mr Greaves said. "Anti freeze is very toxic, very lethal and works very rapidly."
The two families lived about two kilometres away from each other, he said.
While Mr Greaves thought the slug bait deaths could be malicious, he said there could also be plausible explanations such as carelessness by neighbours.
He said at this time of year more people tended to put anti-freeze in their radiators and most failed to clean up properly after themselves.
A fifth cat nearly died and needed a blood transfusion last weekend after it appeared to have eaten large amounts of rat poison, Mr Greaves said.
Sergeant Paul Cade said the Henderson family had reported the deaths. The SPCA is investigating.