McCullum, in Christchurch for the birth of his third child tomorrow, made a brief appearance at a press conference, where he said he would continue to assist the ACSU in its investigation.
The New Zealand captain said he was disappointed his testimony was leaked to media but he would continue to cooperate with the ICC in its on-going investigation.
"I'm obviously disappointed about that but there's nothing I can do about it.
I can't go into the specifics of what's going into the investigation or my involvement in it ... but I will continue to fulfil my role in the investigation.
"I guess from my point of view the dealings I've had with the group that I've dealt with, I have confidence. How the leak happened, I'm not sure but I have confidence in them."
McCullum said he was warmed by the support he had received from around the globe since his testimony was leaked, with the backing encouraging him to continue in the fight to stamp fixing out of cricket.
"There's still a long way to go. Obviously it had been a number of years and the next little while will probably be a bit tough but my role in the investigation is on-going.
"The sport of cricket is a great sport which we're all very privileged to be involved in. Obviously there are a couple of circumstances that have tainted the game but the majority of people uphold the traditions of the game."
As captain of the New Zealand team, McCullum said his advice would be simple if any Kiwi cricketers came to him to report an approach of match-fixing.
"There was no hesitancy in my faith in the ICC as such. If one of the players found themselves in the same situation then I would certainly encourage them to go the same route I chose."
There was only one small moment of discord. McCullum was asked why there was a delay between the alleged approach by Cairns and his reporting of it. McCullum appeared keen to answer but was cut off by New Zealand Cricket chief executive David White and players' association manager Heath Mills, who flanked him during his appearance.
With McCullum's lawyer Garth Gallaway hovering stage left, perhaps it had been decided nothing would be left to chance.
"Sorry, the ICC came forward yesterday and are very comfortable with the way Brendon has conducted himself," White said.
It is understood by the Herald that there is frustration that the time it took McCullum to be interviewed by the ACSU is being misconstrued by some media as the time it took him to report it.
It was reiterated several times this week by both NZC and the ICC that McCullum had done the right thing in his reporting of the approach.
McCullum is likely to be interviewed again. Cairns, who has been interviewed by the Metropolitan police but not the ACSU, will surely face more questioning. This is shaping as the moment of truth.
"I have no understanding why he would say the things he is alleged to have said," Cairns said in his statement this week. "To be clear, I have never approached Brendon, or anyone else, about match fixing or any other improper activity."
McCullum's one-word corroboration of his testimony today has put them on a collision course that seems certain to end in a proper court, not the one of public opinion.
Match-fixing investigation timeline
Dec 5: Trio of cricketers under investigation revealed
Dec 5: 'My heart sank' - Cairns
Dec 6: Cairns' legal team strikes back
Dec 8: Fixing won't die off - Modi
Dec 9: Cairns: I'm still in the dark
Dec 14: ICC probes Aces match in South Africa
Feb 9: Cairns hits back
Feb 10: 'Who's putting this in the media?' - Cairns
Feb 16: Experts say loss of income sparks Cairns counter
Feb 27: Vincent admits approach by bookmaker
Mar 27: Cairns' lawyer arrested
May 15: NZC confirm investigation into Auckland games
May 16: McCullum said no to fixer
May 16: Vincent offered cash and a woman to fix
May 17: O'Brien unsurprised at Vincent revelations
May 18: Friends fear for Vincent
May 20: Cairns is player named as Player X
May 20: Cairns denies Player X allegations
