New Zealand captain McCullum reported two approaches - one involving an offer of more than £100,000 to agree to help 'spot-fix' a match - to the ICC's anti-corruption and security unit (ACSU).
His evidence, dating back to events in 2008, was publicised worldwide this week after a report in the Daily Mail.
However, he insists he is still sure he did the right thing, he would do so again and encourage others to as well if similar circumstances arose.
Asked about his reaction, on discovering his written evidence to the ACSU had been leaked, McCullum told a joint press conference with New Zealand Cricket: "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.
"How the leak happened, I'm not sure, but I have confidence in them."
McCullum does not anticipate a quick resolution to a case allegedly involving a high-profile cricketer - 'Player X' - whom he and former team-mate Lou Vincent have named to the ACSU.
"There's still a long way to go," McCullum said.
"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.
"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."
In a prepared statement, McCullum made it clear - as has New Zealand Cricket and the ICC previously - that his intentions were in good faith and accordance with awareness and education programmes in place to help professional cricketers stamp out corruption.
"Let me be clear - I have never ever fixed a cricket match," he said. "I am 100 per cent behind the fight against corruption in the sport.
"While it's been a tough time for me, I know I did the right thing.
"I was pleased to see the ICC come out yesterday and commend me for my actions.
"It takes courage to report an approach, and it is vital that players who do so feel supported.
"I gave my testimony to the ICC in good faith, so I was shocked when I read it in the media.
"I hope it does not stop others coming forward in the future - as all players need to play their part in stamping out corruption."
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