When asked whether he stood by his testimony to the ICC's anti-corruption unit (ACSU), McCullum was unequivocal.
"Yes."
Player X has been identified by Chris Cairns, a fact he conceded in later tweets. However, Cairns maintains the allegations, seen in leaked testimony from McCullum and Lou Vincent, are "lies".
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 ongoing 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 fulfill 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 ongoing.
"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 certianly encourage them to go the same route I chose."
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
