Campbell's aggressive interview just before last year's election earned him the label "that little creep" from the furious Prime Minister.
The authority criticised TV3 for failing to disclose to Helen Clark the source of its claims - author Nicky Hager, who wrote Seeds of Distrust.
It said Campbell had also failed to "play the Devil's advocate" and challenge Hager's views in the July 10 broadcast.
The authority found:
* The programme was unbalanced on the issue of Government accountability.
* Allegations were put forward as fact.
* Helen Clark and Hager received different treatment.
TV3 said last night that it was considering an appeal to the High Court.
"I have some regrets but they are not matters of substance," said Campbell.
"I don't think my interview with the Prime Minister was the most sophisticated I've done."
He believed TV3's coverage of the Corngate story was accurate and fair.
"The facts are that corn that the Government believed at first to be genetically contaminated came into the country and the Government set about covering it up."
The Government strenuously denied any cover-up and later said if there had been any contamination it was at such low levels it was scientifically impossible to detect.
Campbell said: "I looked at every single email, Cabinet minute and internal piece of correspondence ... I was utterly convinced ... that what we said transpired, did." So there was no reason to put Hager through the ropes.
Helen Clark declined to comment on the decision.
The Prime Minister, her press secretary, Mike Munro, and other parties had complained to the authority that she was hijacked by Campbell. She had no idea beforehand what the interview was about.
The authority found there was no ambush and although the interview was forceful and Campbell persistent, Helen Clark had made her points firmly in a robust exchange.
But it also found that the style of questioning had differed between Hager and Helen Clark, and viewers could have been left with the impression that the Prime Minister was being evasive.
The decision asked the parties for their views on punishment, ranging from nothing to apologising on-air.
Mike Jennings, TV3 director of news and current affairs, said the main concern was the authority's contention that the network should have revealed its sources to the Prime Minister.
"We were not prepared to do that in this case, nor will we at any point in the future."
Clark furious over GE ambush
Herald Feature: Genetic Engineering
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