National Party president Peter Goodfellow leaves the National caucus room during any discussion about the fishing industry to avoid any conflict of interest, Prime Minister John Key says.
Mr Goodfellow has significant interests in fishing company Sanfords, which holds about a quarter of the total fishing quota in New Zealand.
Mr Key was asked about National's closeness to the commercial fishing industry today after a contract to monitor the commercial fleet was awarded to a business whose general partner was owned by fishing companies - one of which is Sanfords.
Speaking to reporters at his weekly press conference, the Prime Minister said Mr Goodfellow was "immensely careful about managing any conflict of interest".
"I've never had a discussion about fisheries matters," he said.
"If they are raised at the caucus, because he goes to caucus, he always leaves the room. He's obviously not a member of Cabinet so he doesn't get access to that information.
"So he treats that conflict of interest very seriously and very professionally."
The Government awarded the contract to set up video cameras on fishing vessels and monitor the footage to Trident Systems, a subsidiary of Sanfords and Moana Pacific Fisheries - a move described by Greenpeace as "the fox guarding the henhouse".
Trident says it is contracted to "gather specific observational data from the footage", which is also accessible to the Ministry for Primary Industries (MPI).
Mr Key defended the decision to give the contract to Trident, saying it was won in an open, competitive tender process.
More importantly, he said, MPI held the information from the video cameras and was able to access it any time.
"While Trident provide a summary to MPI because that's convenient, MPI have access to all of that unbroken data any time they want."
Two weeks ago, MPI launched an inquiry into the decision not to prosecute New Zealand fishing boats for illegally dumping healthy fish in 2013, despite being caught on camera.
Labour's fisheries spokesman Rino Tirakatene now wants the inquiry to be expanded to look at the awarding of the contract to Trident.
The relationship between the ministry and the industry was "too cosy", he said.
Mr Key ruled this out, saying that the contract went to the company which had the best technology and offered the best value for money.