A jury deciding on a lawsuit against former Conservative Party leader Colin Craig has been unable to reach a decision in its first day of deliberations.

Taxpayers' Union executive director Jordan Williams is suing Mr Craig for defamation over what he says were damaging claims made in a pamphlet Mr Craig sent to about 1.6 million homes and at a press conference in 2015, following Mr Craig's resignation as leader amid rumours of sexual harassment.

Mr Williams says Mr Craig accused him of spreading lies about the reasons behind the sudden resignation of press secretary Rachel MacGregor two days before the 2014 national election.

He says he was warning the party after Ms MacGregor confided in him and has asked a jury to award him $1.4 million in damages.

On Thursday, the 11-person jury in the trial retired to begin deliberating on whether Mr Craig had defamed Mr Williams and, if he did, how much the payout should be.

During the day the jurors came back to watch two videos of Mr Craig, but were unable to reach a verdict by the evening.

The case is the first jury trial for defamation at the High Court in Auckland since 2002 and the transcript of the four weeks of evidence is more than 1000 pages.

The number of questions the jurors have to answer is possibly the most of any defamation trial ever in New Zealand.

Mr Williams told the court he went to senior Conservative Party officials after Ms MacGregor confided in him about alleged sexual harassment by Mr Craig, including touching, comments, and romantic letters and poems.

But Mr Craig says Mr Williams spread damaging and untrue rumours, including to blogger Cameron Slater, and the pamphlet was written in self-defence.

Summing up the case for the jurors, Justice Sarah Katz told the jurors they had to decide whether Mr Craig's comments had lowered people's opinion of Mr Williams and, if so, whether the comments were true, made in self-defence or were just honest opinion - the three defences raised by Mr Craig's team.

But she said if Mr Craig had written the pamphlet with the main intention of just hurting Mr Williams, it would lose its protection - and the jury would have to decide whether they believed Mr Craig about his motivation.

"You have heard two very different versions of events unfold in the courtroom ... It is for you to navigate your way between the conflicting versions of events and decide where the truth lies," she said.

The jurors will return on Friday.