The Otago SPCA is investigating the incident after receiving a call from the concerned citizen who made the discovery while walking on the beach on Tuesday.
The carcass was found on top of a small extinguished bonfire.
Otago SPCA senior inspector Steph Saunders said staff were hoping witnesses to the bonfire would come forward with more information.
"While this was a gruesome find, the issue for SPCA is whether the cat was dead or alive at the time of being burnt."
At this stage, the SPCA could not rule out the possibility that the cat was already dead, she said.
"[But] if alive at the time, the cat would have suffered an excruciating death."
It appeared an accelerant had been used and the SPCA had taken samples from the carcass.
"We are confident of having a better idea of what actually happened from those results."
The SPCA believed the fire, just above the high-tide mark, was most likely lit on Monday evening and would have been visible to Tomahawk residents with a view of the beach.
While staff were following up some leads, the SPCA would like to hear from anyone who witnessed this event or noticed anything unusual in the Tomahawk area on Monday or Tuesday, she said.
"We have had a gutsful of people lacking respect for animals."
If it was proved the cat was alive when put on the bonfire, the offender could face prison, she said.
A man was prosecuted in Dunedin several years ago for killing a cat that wandered on to his property and putting it in his incinerator.
