"There are so many moral issues that come up when you are doing journalistic things, when you're intruding in people's lives and judging things and criticising things, there are endless moral issues. It's huge, it's a big part of the job," he said.
"The first thing I did [when researching for Dirty Politics] was went to my lawyer Steven Price and said, I might be able to get a whole pile of information which I would like to use which has been hacked, and if I do this will I be in trouble? He checked on it and ran through some things and he said 'If you use the information you've been given you are not breaking the law'. So legally that was fine.
"But there are stacks of other ethical issues and the biggest one was when you get leaked a pile of someone's emails of course maybe a third of them are about politics but a whole lot of them are about stuff that are none of my business. They're about private stuff. It's good to have these hard crunchy issues, but what's the difference between having something that's private and that's secret? There's a very important difference. You can have some secret stuff that the public has the right to know, and you can have someone's private stuff and they're completely different categories," he said.
"If you do a high profile, owned up to hack, and then get caught, you will probably go to prison for years. This is very serious. These are high stakes. But then again, it's high stakes if politics is being dominated by personal attacks and phoney scandals that have originated from the beehive and sent out through people in sneaky underhand ways."
Amid rumours circulating in the wake of the sold-out book, was that the source, was said to be another politician of German decent. Hager put that one to bed.
"The first thing that happens when you do something that people don't like is they try to attack you. The attack that was used against me was that my source was Kim Dotcom. And why did they choose Kim Dotcom? Basically, because he's bogie of the moment for the right, so if I was in control of an agent of Kim Dotcom then that would smear what I've done.
"When the book came out there were some non-credible comments from people like John Key saying, 'he's just doing it to get the money', which means that John Key knows about many things but he doesn't know about the book trade," he said.
With all the risks, all the personal attacks, and according to the polls, little impact on the National Party's position, what did Hager hope to achieve?
"The thing that holds us together as a society is that we know what's going on. The most important journalism is the stuff which means that day by day we know what people are saying, we know going on in politics and we know what the government decisions are on things.
" A whole lot of people have done things that are dodgy and wrong. There were people I didn't name [in Dirty Politics], they were on the press gallery and senior journalists. But I didn't want to humiliate them, I wanted to give them room to think and do it differently.
"I've only got one life, there's hardly anyone in New Zealand that does this stuff so I would rather get my modest income and my total freedom. That's what I choose.
"Why have I chosen a job where after 20-something years will mean that I've earned less than any of you in your first job? The reason for that is that I so enjoy it. It's so satisfying. And this is coming down to something, which should be regarded as dodgy end journalism, so why would you do it? But the main reason is, it's like police officers or people that work in hospitals, they're doing something useful to serve society. It's for the people, and not just yourself.
"There but for the grace of God go I."