South coast snorkellers and divers are always excited to see Blue Gropers appear among coral and kelp.

The species - which are NSW's state fish emblem - are known for their calm temperament and inquisitive nature, often approaching divers for food or a look around.

However, for one Sydney fisherman, his exciting experience with a Blue Groper turned to fear when he was told the giant fish he had just speared could cost him a $22,000 fine.

A Sutherland shire resident came upon the spearfisherman and his "catch" at Hungry Point, Cronulla when she took her son fishing.

"I took a photo, not realising at the time what he he had killed," she said.

"When I found out I went back and told him it was a Blue Groper and he could be fined up to $22,0000. He took off pretty fast after that and stopped whooping.

"Maybe there should be signs at Hungry Point for spearfisherman who don't know, or need reminding what they are lawfully entitled to kill and not kill.

"Tragically, it would be too late for this gentle giant.

"It's a shame there were no fisheries licencing officers around."

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A spokeswoman for NSW Department of Primary Industries said gropers were protected from commercial fishing and spearfishing.

"Gropers can only legally be taken by line in NSW," she said.

"For recreational fishers, a minimum size limit of 30cm and a bag limit of two (with only one fish over 60cm) applies.

"Penalties may include a $500 on-the-spot fine per offence for taking a groper by any method other than a line or handline, possession of prohibited size or exceeding the bag limit.

"Maximum penalties by way of court prosecution for an individual are $22,000 and/or six months imprisonment for a first offence relating to size and bag limits, and $11,000 for taking groper by an unlawful method."

For more information about recreational fishing rules visit dpi.nsw.gov.au/fish.../recreational/fishing-rules-and-regs