Investigative journalist Nicky Hager has reluctantly told a court the kind of media attention Colin Craig received after a poem he wrote to his press secretary was published online was typical of the way blogs are used to attack political candidates.

LISTEN ABOVE: NZME senior police reporter Anna Leask has been at the trial and spoke to Larry Williams

Taxpayers' Union executive director Jordan Williams is suing former Conservative Party leader Mr Craig for defamation over comments made in a pamphlet sent to 1.6 million homes and a press conference in 2015.

He says Mr Craig accused him of spreading lies about the reasons behind the resignation of Mr Craig's press secretary, Rachel MacGregor, days before the 2014 national election as part of a strategy to role him as leader - including by leaking details of allegations to blogs.

Mr Williams says he went to senior party officials out of concern after Ms MacGregor confided in him about alleged sexual harassment by Mr Craig, including touching, comments, and romantic letters and poems.

On Wednesday, "Dirty Politics" author Nicky Hager told the court the way information had been released about Mr Craig was typical of how blogs were used in attacks by political groups.

He started by saying he had not wanted to participate in the trial, describing the case as "tawdry", but had been subpoenaed to appear by Mr Craig's lawyers.

Putting anonymous claims on blogs would allow political agendas to be hidden and would sometimes lead to stories breaking in mainstream media when they would otherwise would be considered too personal in nature, Mr Hager told the jury.

Mr Williams had given blogger Cameron Slater one of Mr Craig's poems to publish, "predictably" leading to a number of media companies picking the story up, Mr Hager said.

"Mr Williams knew in doing so, a story would be published in the mainstream media."

Earlier former party chair Brian Dobbs told the court he had first grown concerned about the level of closeness between Ms MacGregor and Mr Craig after the 2011 nation election and spoke to Mr Craig multiple times about staff relationships until 2014.

Mr Dobbs said he felt Ms MacGregor's resignation was partially responsible for the party narrowly missing out of parliamentary seats in the 2014 election.

The case continues.