Fishers have been warned if they set nets in restricted areas off the west coast of the North Island they will go to court and could lose their boats, their gear and towing vehicles.

The warning has come from the Department of Conservation and the Ministry of Fisheries as part of their campaign to save the critically endangered Maui's dolphins.

It also followed the discovery of a 60-metre drag net earlier this week off Karioitahi on the west coast, south of the entrance to Manukau Harbour.

The net was lost by a fisherman. Department of Conservation staff and Karioitahi lifeguards recovered it but DoC spokesman Phil Brown said the net was a big worry.

"Monofilament nets can keep on catching if they are left to float around. They can and do catch dolphins as well as sea birds, seals and other marine life. Fishers are asked to make sure they recover any net they put into the water."

He said only about 100 Maui's dolphins were left and people who ignored the rules were putting the dolphins at serious risk.

"Maui's dolphins are perilously close to extinction and everybody needs to do their part to help protect the remaining population."

Because of the danger they posed to Maui's dolphins, set nets were banned in 2003 within four nautical miles of the coast from Maunganui Bluff, north of Dargaville, to Pariokariwa Point, north of New Plymouth. The ban also extended into the Manukau Harbour.

Amateur fishers found fishing with set nets in the closed area could be fined up to $20,000 and have their fishing equipment seized. Commercial fishers could be fined up to $100,000.

Maui's dolphin is the world's smallest dolphin and New Zealand's rarest. It is a close relative of the Hector's dolphin which is found off the South Island.