UPDATED 1.51PM Colin Craig's former press secretary Rachel MacGregor is to give evidence against him at a High Court defamation trial later this week, revealing the reasons for her sudden departure from her role before the 2014 general election.

MacGregor stood down two days before the election, accusing Craig of being "manipulative".

She has never given any further detail about her specific reasons for leaving but is set to reveal all of the details this week in the High Court at Auckland.

MacGregor will be giving evidence in support of her friend Jordan Williams' defamation case against Craig.

The leaflet was titled Dirty Politics and Hidden Agendas, in which Craig outlined what he saw as the "strategy" to remove him as leader of the Conservative Party.

The trial started yesterday before Justice Sarah Katz and Williams' lawyer Peter McKnight delivered his opening address to the jury this morning, outlining what he said was going to be a "complex" case.

McKnight said MacGregor's description of Craig was accurate, and he is in fact "a manipulative man".

"He is a person who will use his power and wealth to, in effect, manipulate and control others," he said.

McKnight said the alleged defamation came about after MacGregor left Craig's employment and struck up a friendship with Williams.

She confided in Williams about what had led to her departure, and what happened in the aftermath. Williams was shown letters and text messages written by Craig to MacGregor, and was significantly concerned about the situation.

MacGregor had taken a complaint about Craig to the Human Rights Commission and the pair had attended mediation and come to a settlement agreement.

Part of the agreement was that the details stayed strictly confidential, that neither party could speak about the case, other than to say that the matter had been resolved.

However in a news interview Craig, who appeared in a sauna with a TV3 journalist, answered questions about MacGregor that she believed breached their confidentiality agreement.

Williams was concerned to such a level about Craig's conduct that he decided to share the information with several other senior members of the Conservative Party.

In response, Craig held a press conference and inferred Williams had lied about him.

"In short he suggested... Jordan Williams was dishonest and is a party to a malicious campaign to spread defamatory lies about Colin Craig," McKnight told the jury.

Williams' claims Craig was behind a "campaign of defamatory lies" against him and he is seeking general and punitive damages totalling more than $300,000 in relation to the comments at the press conference and a further $600,000 in relation to the pamphlet.

The trial continues.