Hager's first book, Secret Power, was published in 1996.
"I've been going about 25 years. I got into it, not through a journalism degree, but by writing my first book," he said.
"Research is just tremendous fun. Working out a project, planning how to find sources, finding people who are prepared to be confidential and provide information ... that part of the job hasn't changed at all."
But the advent of the internet, falling media revenue and the rise of citizen journalism and blogging signals big changes in the field.
In the fallout over Dirty Politics, Hager has found himself in court defending his right to keep sources confidential. He said the idea of journalistic privilege was about a person's conduct.
"To my way of thinking, the way that you tell a journalist is not by them having a paid job in a media organisation - that's going to be a shrinking definition - it's about people who are prepared to work by standards and are acting independently."
Despite many predicting the demise of the craft, Hager doesn't see it that way.
"Investigative journalism is something the public want - it will just keep on happening."
The NZ Investigative Journalism Conference had almost doubled its attendance in the past two years, he said. The financing of such journalism had a different outlook, however, and required passionate people to be involved and support each other.
"It's totally bleak, but what I say is this: in New Zealand, we are never going to have lots of money for these things. In New Zealand, we shouldn't have authors or dancers or actors because they're just not financially viable, but the way it works is people want to do it and they find ways to do it."
-Nicky Hager will speak on September 18 at 7.30pm at the concert chamber in the War Memorial Centre.
