It was this which led to one inquiry, in which Mrs Collins was cleared.
The second inquiry uncovered collusion between the Prime Minister's office and Slater over the release of classified information used to embarrass Labour's Phil Goff during the 2011 election.
Labour leader Andrew Little said it revealed "the dirtiest, filthiest, grubbiest, vilest operation we have ever seen in New Zealand politics".
Mr Key said he had been cleared.
Whatever Dirty Politics alleged, there was no escaping it was based on material which was also highly dubious in nature. Slater's computer had been hacked and its contents delivered to Hager for his use.
The police carried out a 10-hour search of Hager's home, removing notes and computer equipment in the search for his source, the hacker Rawshark. The search is currently the subject of two High Court actions.
Slater has continued blogging, saying Hager's book was part of a left-wing conspiracy intended to drive him to suicide. He has blogged that he will reveal the "real dirty politics" in his own book and through an upcoming media venture called Freed, funded by tech millionaire Tony Lentino.
Nicky Hager - For journalism, publishing Dirty Politics and shining a light.