McCullum is currently caught in a legal battle, with a High Court injunction put out to stop a newspaper in New Zealand from publishing confidential information involving him and Kerry Schwalger, a mental skills coach, according to The New Zealand Herald.
McCullum has been interviewed by the International Cricket Council's (ICC) anti-corruption unit and a leaked report revealed he claimed to have been approached for fixing matches by a former team-mate, dubbed "Player X".
Cairns later admitted that he was "Player X", but strongly denied the allegations and he was spotted with a plastic folder that had the names "McCullum" and "Schwalger" on it, shortly after he faced perjury charges in England.
McCullum's lawyer Garth Gallaway said despite efforts to prevent the media publishing confidential material involving the Black Caps skipper, he remains determined to give evidence if it means the sport will be better for it.
"Ironically, Brendon remains unwavering in his desire to see the game be rid of match-fixing," Gallaway told the Herald. "His resolve to give evidence, if he is required to do so, in relation to the match-fixing approaches made to him is stronger than ever."
McCullum, for his stance, has received support from chief executive of New Zealand Cricket David White, who however refrained from commenting on specific matters before the courts.
"New Zealand Cricket supports Brendon McCullum. In particular, we support his right to privacy and client confidentiality, and we support his right to bear witness," White said.