Authorities warn of stern action against those caught illegally catching mud crabs in New South Wales.

With the crustaceans selling at $70 a kilogram, it is proving a lucrative crime for those offloading onto the black market.

The NSW Department of Primary Industries' director of fisheries compliance Patrick Tully said the area of most concern was unlicensed fishermen.

"It's illegal, unregulated and unreported," he said.

"We're concerned that there are people using too many crab traps, not marking them so they can't be found, and then selling them on what's essentially the black market."

"At $70 a kilo one crab could be more than $70, they can grow to quite big animals.

"It's what we used to call the 'shamateur' — not a licensed fisherman, not really a recreational fisherman, just that person in the middle who is exploiting the resource at the expense of others."

Mr Tully said the illegal activity of unlicensed trapping would be targeted in the lead up to summer holidays.

"The warmer months when there are more people around there's more of a premium on the seafood, Christmas especially," he said.

"That makes it even more lucrative to get out and about, and with the longer days and better weather you can see how that happens."

Action has already been taken against a number of "shamateurs" caught.

"We've seen a couple recently where we've taken the reasonably extraordinary measure of taking someone's boat off them and having that forfeited to the Crown," Mr Tully said.

"They were clearly engaged in that kind of behaviour: unmarked traps all over the place and putting together a stockpile of crabs to sell on the black market.

"There are ongoing operations I can't talk about but just recently a couple of our fisheries officers went into a fish market and saw a couple of boxes of black crabs, mud crabs, on the floor and which to the uninitiated would appear to have been legally commercially caught.

"But upon further investigation, and using the intelligence the officers have, we found that they weren't in fact caught appropriately. They were being black marketed so we've taken action against that."

A Department of Primary Industries spokesman said penalties for a first offence of unlawful use of crab traps ranged from on-the-spot fines of up to $1,000 to a maximum penalty of $22,000, six months jail, or both.

Penalties doubled for second and subsequent offences, and illegally used traps would be seized.