The Ministry for Primary Industries (MPI) says it has received over 160 calls and emails so far in 2017 reporting Facebook posts by people illegally selling recreationally caught seafood.
It says this is well up on 96 complaints received in 2016 and 57 in 2015.
"Both buying and selling recreationally caught seafood, as well as swapping it, is against the law. These are offences against the Fisheries Act 1996 and can result in fines of up to $250,000," says MPI Fisheries Compliance Manager Greg Keys.
Mr Keys says it's likely that most people who are posting about seafood for sale know that their actions are illegal.
"It's always disappointing to see people flouting the rules. What is great, however, is an increase in the number of people who approach us via our various channels to report suspicious or illegal behaviour," he says.
"Ensuring and promoting sustainable fisheries is a collective responsibility. We encourage people to report poaching or illegal activity by calling 0800 4 POACHER (0800 47 62 24) or emailing firstname.lastname@example.org or using our main Facebook site."
All correspondence with MPI is treated as confidential.