Fishery officers are investigating after about 100 dead snapper were found floating in the Hauraki Gulf on Saturday.
The Ministry for Primary Industries said Auckland officers wanted to talk to anyone who saw the dead fish, which were reported around 2pm.
The fish were floating in the area north of Waiheke Island.
"At this point we are in very early stages of our inquiries and are appealing to the public to report any activity that could have contributed to the dead snapper," MPI said in a statement.
"We are very thankful to members of the public for contacting MPI."
Anyone with information about this incident - or any other suspicious fishing activity - is urged to call 0800 4 POACHER line (0800 47 62 24) or email email@example.com to report it.
The incident follows the death of thousands of mackerel last month, with the dead fish found washed up across the shores of Kawau Island.
MPI later said it believed the fish - about 1000 kgs' worth - had accidentally been lost from the net of a commercial fishing vessel.