Colin Craig's former press secretary Rachel MacGregor is to give evidence against the former leader of the Conservative Party today at a High Court defamation trial.

MacGregor will read her evidence to a jury in the High Court at Auckland from 11am today.

It will be the first time she has spoken publicly about Craig and her sudden resignation from her press secretary role just days before the 2014 general election.

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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 resignation.

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.

Craig then 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 that Williams was a "liar" had and "spread false accusations".

Colin Craig (right arrives at the Auckland High Court. Photo / Brett Phibbs
Colin Craig (right arrives at the Auckland High Court. Photo / Brett Phibbs

Williams then filed defamation proceedings in the High Court, saying he did not lie about Craig.

MacGregor is giving evidence in support of Williams' allegation.

It is expected Craig's legal team will then cross examine MacGregor on her evidence.