A 26-year-old Kiwi is being fingered as the culprit behind the killing of Sydney’s most beloved fish.

Divers and locals alike have been left distraught after the decades-old protected blue groper was speared by a fisherman in front of beachgoers inside a marine reserve.

Police say the man responsible for the slaughter was a 26-year-old from New Zealand, according to the Daily Mail. 

New South Wales marine police were called to Cronulla’s Oak Park on December 30 after reports of alleged illegal spearfishing.

The fisherman was questioned by police and fined $500 for killing the fish in a no-fishing zone.

The Daily Mail said moments after the killing, appalled locals saw the man, his wetsuit stained with blood, posing for a trophy picture with the slain 40-year-old fish.

Oak Park local Tenille Piek said her parents witnessed the shocking event.

Piek said the man pulled the groper out of the water “triumphantly” and showed off his killing without realising he could be fined.

“My mother was the first person to approach the man requesting to take his photo,” she said.

Her family had swum with Gus for 30 years.

Another local who saw the event unfold claimed the fisherman was “hurling abuse at locals” as he walked past him.

Others have described the dead marine giant as “amiable” and a “true companion” who swam alongside divers exploring Oak Park.

“Gus, you were more than just a diving buddy. You were a true companion, joining me on countless dives at Oak Park, Cronulla,” Abyss Scuba Diving posted on Facebook.

“Your untimely demise on December 30, 2023, at the hands of a spearfisherman has left us devastated.”

NSW Police said officers were unable to locate the groper but it was believed it had been returned to the sea.

Locals have proposed the area at Oak Park in Cronulla where the groper was killed to be named Gus Reserve as a tribute.

Sydney locals have paid tribute to Gus the blue groper after it was killed by a Kiwi fisherman. Photo / Abyss Scuba Diving 

The blue groper has been the premier state’s official fish for more than 25 years.

The endangered species is protected and cannot be caught commercially or by spear.

Spearfishing of the blue groper fish is prohibited and the maximum fine penalty is $22,000 or six months in prison.

The penalty does not require an appearance in court and therefore the man’s name has not been publicly released.

The Department of Primary Industries told another news service the man showed “significant remorse” when he was confronted by fishery officers.