A judge has ruled Colin Craig can go ahead with one of his main defences against defamation after he forgot to include it in his case.
Rachel MacGregor worked as press secretary for the then Conservative Party leader between 2011 and the 2014 election, but resigned two days before polling.
She lodged a sexual harassment claim and there was later a confidential settlement.
Mr Craig is suing Ms MacGregor for defaming him on three different occasions including in details she gave Jordan Williams from the taxpayers Union about their relationship.
Ms MacGregor is counter-suing Mr Craig for defaming her four times, including in a booklet sent to 1.6 million households.
During the first day of proceedings Mr Craig, who is representing himself, sought to add an argument of truth to his defence against her allegations, saying it was an error not to have included it originally.
"Obviously I argued in my opening submissions on the grounds of truth and my friend's pointed out that's not within my pleadings ... that's a mistake on my part and it ought to be in my pleadings."
There are a number of defences against defamation, including whether what is said is true.
He said it was an "oversight" and he had not realised Ms MacGregor's lawyers had pointed that out in their submissions.
"I don't know that it does create prejudice as such and a lot of my evidence goes to the defence of truth in any case."
Ms MacGregor's lawyer, Hayden Wilson, opposed the change.
"Mr Craig is not a man who is new to defamation proceedings. He has never pleaded truth in this proceeding to date.
"I'm a little at a lost that when we served our opening which was explicit, that Mr Craig didn't realise this was a mistake at that point.
"It's a substantial amendment by any means to introduce a new defence after the opening of the claim when there's been no new information that may give rise to it.
Justice Hinton gave counsel until this morning to consider the matter.
This morning Mr Craig directed her to part of his submission in which he said his "main defence" was truth.
"Oh, I see your main defence is that the allegations you made were true. Your main defence is just not pleaded," Justice Hinton responded.
She granted his amendment.
Mr Craig is giving evidence today in the hearing which is expected to take two weeks.