"I can't," Craig said.
He assured the jury that he was "pretty sure" of the nature of the messages he had sent MacGregor.
"There was never anything vaguely like a sex message," he said.
Craig is being cross-examined in the High Court where he is on trial for allegedly defaming Williams, the Tax Payers' Union director.
Williams' lawyer Peter McKnight has spent the day going through a pamphlet in which Craig allegedly made the defamatory comments.
Line by line he has quizzed Craig on the background, context, meaning and intent of the content.
Craig's "wonderful" poem dissected for jury
McKnight earlier went through a poem Craig had written about MacGregor line by line and asked him to explain what he meant by his words.
Craig said the line "You are wonderful because you make me smile" was written because MacGregor was "an eccentric person".
"She was a slightly quirky person," he said.
The line "You are wonderful because you think like me" meant that Craig felt he MacGregor were like-minded and had the same outlook on life.
The line "You are wonderful because you don't think like me", according to Craig, meant MacGregor "wasn't backwards about coming forwards".
McKnight asked Craig what the line "Beautiful (Please skip this section if inappropriate)" meant.
"At the beginning of the letter I made it clear it was to encourage her and make her feel valued and she really appreciated me doing that," Craig said.
"I understood that this was right on the boundary."
He agreed when McKnight asked him if he now thought that line was inappropriate.
McKnight carried on, having Craig read more lines of the poem.
"Your eyes are lovely. You look unbelievable in your new dress."
Craig said MacGregor did have lovely eyes and the line meant "what it said".
He explained that he complemented her dress as it was a new purchase and she "showed it off" and asked him what he thought.
The next line of the poem read "Your lips are so amazing to kiss".
"What did you mean by that?" McKnight quizzed.
"At the time I wrote it I was trying to encourage and build up... it was obviously inappropriate and it went too far," Craig responded, adding that MacGregor was attractive and he suspected many people would want to kiss her.
"I did once..." he said referring to the time the pair kissed on election night in 2011.
Craig said the poem was "endearing".
"At the same time I acknowledge it was too close, too personal and I accept that."
McKnight asked Craig if the poem showed "lust".
"You have the most perfect... what does that mean?" McKnight asked.
Craig said he dotted off the line to let MacGregor fill in the blanks.
"I didn't fill that in deliberately, I didn't necessarily have anything in mind when I wrote that. What I was trying to do was get her thinking that she was a beautiful person," Craig said.
"I remember at the time thinking nooo, this is as far as I should go."
The next line read (LOL ... ok I deleted a couple of lines and stopped this section).
McKnight asked what the deleted lines were and if they were sexual.
"No," Craig said.
"Are you being honest with this jury?" McKnight pressed.
"Yes."
Craig told the jury that he was "close friends" with MacGregor until she resigned, then his view of her changed dramatically.
"When she resigned, that was unbelievable that she would do that," he said.
"She has two versions of events in her mind. One is Colin who was fun to work with... paid for her to spend a day at a day spa... gave her a loan... she has another version of events of me which is dodgy and disgusting.
"I don't believe that that is correct. I think her first version of events is the right one."
Why is Colin Craig on trial?
The defamation trial unfolded after Craig's press secretary, Rachel MacGregor, resigned suddenly just 48 hours before the 2014 general election.
The resignation was high profile and there was much speculation about why she left.
Weeks later MacGregor turned to Williams for support, and told him she had made a complaint to the Human Rights Commission that Craig had sexually harassed her.
She shared letters and poems the politician sent her. Williams then revealed the details to other Conservative Party members.
When Craig found out he publicly claimed Williams was part of a group of "culprits" determined to have him removed as party leader through a "campaign" of "false accusations".
Williams then filed defamation proceedings in the High Court, saying he did not lie about Craig.
After Craig finished reading his brief of evidence to the jury last week, McKnight started the cross-examination.
The trial continues.
