A man has been fined $500 for spearing a protected fish species in Sydney over the weekend.

NSW police have confirmed officers spoke to a 26-year-old man on Saturday following reports a blue groper (Achoerodus viridis) was speared and killed at Oak Park in Cronulla.

Police said the fish was not found but is “believed to have been returned to the water”.

According to local reports, the community believes the fish may be “Gus”, a 35- to 40-year-old blue groper who is well-known to swimmers and divers in the area.

“Following inquiries, the man was issued a penalty notice for take groper not use rod and line or handline which carries a penalty of $500,” police said in a statement.

On Monday, the NSW Department of Primary Industries (DPI) said it was investigating after it received a photograph and information regarding the incident.

“Blue gropers are the official state fish and we are disappointed to hear one may have been illegally speared in Cronulla,” a DPI spokesperson said. “Groper are protected from commercial fishing and spearfishing.”

People can catch a maximum of two gropers a day by line only. They must be bigger than 30cm and only one can be over 60cm. Maximum penalties for illegal fishing include a $22,000 fine or six months in jail.

The NSW agriculture and fisheries minister, Tara Moriarty, said on Monday the groper’s death had been “terribly disappointing” for the local community.

“It’s terribly disappointing for the local community of Cronulla’s Oak Park Beach who have publicly and privately shared their sadness at the loss of ‘Gus’ due to illegal spearfishing,” she said.

Blue gropers are known for being peaceful, friendly and inquisitive. They can grow to 1.2 metres long and stand out with their bright blue colouring.