A leak is a safety valve in society - and the Whaleoil hacker is a "decent and ethical person", Dirty Politics author Nicky Hager told a packed hall of people in Auckland last night.
Hager said the attack politics outlined in his book could be combatted by greater public engagement in democracy.
They included the Prime Minister's staff organising personal attacks through the blogger Cameron Slater.
The author was given a standing ovation welcome from the crowd of 350 people and a second of roaring applause after speaking at the Mt Eden War Memorial Hall.
"A leak is what you do when something important is going on that would not be found out otherwise," said Hager.
He said the book revealed a style of politics which was designed to exclude the public. It involved personal attacks which, if successful, would leave the political process the"nastiest, most unscrupulous people".
"I don't believe for a moment politics is awful, shabby and bad and all those things. But it will be if we leave it to the people who are featured in my book."
Hager was introduced by retired appellate judge Sir Ted Thomas, who told the crowd the author had an obligation as a journalist and citizen to make public information a hacker took from Slater's computer.
He said the use of the information was outweighed by the public interest in having it revealed. He said it was "vital in a democracy deviant political practices are exposed".
"In effect, absent the bullets, the Prime Minister's office was hiring a hitman to do its dirty work."