Department of Conservation Rangers and Police swooped on the Porangahau River this week seizing nine whitebait nets for out-of-season illegal fishing.
Conservation Services Manager, Dave Carlton, said the operation took place after receiving tip offs from the public about illegal fishing and selling before the season starts on August 15.
Mr Carlton said he was shocked and disappointed by the level of illegal activity.
"This puts the fishery at risk of decline or even collapse," he said.
"The whitebait fishery is carefully managed to balance breeding success of the fish and harvest by fishers".
The whitebaiting season runs from the August 15 to November 30 except on the West Coast of the South Island which runs from September 1 to November 14.
He said DOC staff would be patrolling fishing sites and talking to fishers to check they are compliant.
Fishing is permitted from between 5 am and 8 pm or between 6 am and 9 pm when daylight saving comes into effect.
"Illegal fishers can be fined up to $5,000 for certain offences so the public are urged to make themselves aware of what is permitted under the regulations."