Brendon McCullum says he will "never forgive" Chris Cairns and he hopes he never again crosses paths with the man he once considered a hero.

McCullum testified against his former teammate in last December's perjury trial at Southwark Crown Court in London, during which Cairns was found not guilty of charges relating to alleged match-fixing.

And, ahead of the release of his autobiography tomorrow, McCullum lashed out at Cairns during an interview with Seven Sharp, saying the fact the court case became "about me versus [Cairns] is something I'll never understand.

"Under pressure, people show their true character. It's not the character I thought I knew, and I guess I can never forgive him for that".

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McCullum, who retired as New Zealand captain in February, said he had no regrets about his decision to testify against Cairns, even if it meant irreparable harm to the pair's relationship.

"I didn't have to testify. I think that's something that people don't understand. And I'm pretty sure lots of people wouldn't do what I did. But I felt I had an obligation, especially as captain of New Zealand, to go over and tell my side of the story.

"I was prepared to stand up, even under pressure and under fire from various quarters, and do what I thought was morally right at the time.

"I'm sure [Cairns] will get on with his life, and I'll get on with mine, and let's hope they never cross paths."