Northland district compliance manager Steve Rudsdale said people selling seafood illegally on social media was generally seasonal in the North.
"The last spate happened during the cray season. We had reports of six people altogether, selling crays [through social media]," Mr Rudsdale said.
"Generally it's people on 'closed sites' so we rely on people who see this to notify us, and take a screen grab if possible, so we can take it further."
He said that during the cray season MPI received reports of a Northland woman selling 12 crays on a social media site. MPI received three calls in five minutes about the sale and went to see the seller 30 minutes later. By that time she had sold all the crays and taken the post down, but evidence secured from her home led to her being charged over the alleged illegal selling. She is still awaiting a court hearing.
"We get reports [of illegally selling online] during the oyster season and, with snapper running from November to December, we are expecting a lot more. But how many we get really relies on the public telling us as soon as they are aware of this happening."
MPI national compliance director Dean Baigent said the online environment had given people a new way of trading blackmarket seafood, but fisheries officers and intelligence staff have been quick to adapt to offending online.
"The pleasing thing is that people are quick to tell us about this sort of activity, no matter where it happens. We get a lot of calls to our 0800 4POACHER hotline about online posts offering cheap seafood. If it's a minor offence, we direct the seller to take the post down and advise them that their name and details are stored by MPI and that similar offending will result in a fine or prosecution," he said.
"If the offending is more serious, we'll investigate with the potential of issuing an infringement notice [fine] or laying charges."