About 10 tonnes of mostly dead fish were found in part of a commercial fishing net drifting 2.5 nautical miles south of Kawau Island on December 13.

A trail of dead fish about three kilometres long was seen, the Fisheries Ministry says.

A ministry patrol boat went to investigate, while the HMNZS Hawea also searched for the net, along with an air force Seasprite helicopter. Both defence units were on exercise in the area. An air force Orion also became involved in the search with crew members taking photographs of the floating fish after it was spotted by the Hawea, the fisheries patrol vessel directed to it. The net was too heavy for the Hawea to lift, so HMNZS Manawanui arrived to take the net and its contents aboard.

RECOVERY OPERATION: HMNZS Manawanui crew members attach winch ties to the drifting net off Kawau Island

Some live fish recovered were returned to the sea, along with most of the dead ones.

The net piece is about 10 metres long and is thought to have been in the water for only a day, Fisheries Ministry field operations manager Greg Keys says.

Investigations are continuing.

"We know nets can rip but it's unusual to find a piece of net that has become separated from the boat," says Mr Keys who has not seen a similar incident in three years. "At this time of year, snapper can be found in large concentrations relatively close to the shore.

"This follows a recent find of snapper off the Northland coast, which turned out to be from a commercial vessel."

Fishers who find floating fish – dead or alive – should record the time and date, along with the location and species of fish and get a sample if possible.

Call 0800-476-224 with any information.