"When the court case started it looked to be very personal and tawdry," he explained.
"I did not want to be part of it anymore."
He wrote to Craig's' lawyers and informed them of this, and in return was sent a subpoena ordering him to appear as a witness.
"Now, here I am," he said.
Hager explained to the jury that his evidence was to give insight into "the use of political blogs by people with political agendas".
He said such people releasing information through blogs, according to his research, resulted in mainstream media outlets picking up on stories they otherwise would not have known about or thought of.
He said Williams leaking a poem Craig had written his former press secretary Rachel MacGregor to Cameron Slater who runs the Whale Oil blog was an example of this.
Hager said after Slater published a blog about the poem, most mainstream media in New Zealand carried and developed the story.
"Nationwide publicity followed... it was a predictable result of the process I described earlier," Hager said.
His evidence was short - less than 15 minutes - and he was not cross-examined by Williams' lawyers.
Craig is on trial for allegedly defaming Taxpayers' Union director Jordan Williams, a friend of MacGregor's to whom she turned after her high-profile shock resignation shortly before the 2014 general election.
Williams said he was "horrified" at MacGregor's claims Craig had sexually harassed her, and after seeing letters and poems the politician sent her, revealed all to other Conservative Party members.
When Craig found out he publicly claimed Williams was part of a group of "culprits" determined to have him removed as party leader through a "campaign" of "false accusations".
At a press conference and in a pamphlet sent to more than 1.6 million households across the country Craig claimed that Williams was a liar and had "spread false accusations".
Williams then filed defamation proceedings in the High Court, saying he did not lie about Craig.
Why is Colin Craig on trial?
The defamation trial unfolded after Craig's press secretary, Rachel MacGregor, resigned suddenly just 48 hours before the 2014 general election.
The resignation was high profile, sparking speculation about why she left.
Weeks later MacGregor turned to Taxpayers' Union director Jordan Williams for support, and told him she had made a complaint to the Human Rights Commission alleging that Craig had sexually harassed her.
She shared letters and poems the politician had sent her. Williams then revealed the details to other Conservative Party members.
When Craig found out he publicly claimed Williams was part of a group of "culprits" determined to have him removed as party leader through a "campaign" of "false accusations".
Williams then filed defamation proceedings in the High Court, saying he did not lie about Craig.
After Craig finished reading his brief of evidence to the jury last week, his Peter McKnight started the cross-examination.
