Exclusive - NZ Cricket boss says captain has right to bear witness.

Black Caps captain Brendon McCullum said he has an "obligation to protect the game" as he prepares for a High Court showdown with former team-mate Chris Cairns.

Cairns will face a perjury charge in London starting on October 5 next year. He's accused of lying during his successful libel trial in 2012, when he sued Lalit Modi after the former Indian Premier League supremo posted on Twitter that Cairns had been kicked out of the rival Indian Cricket League for match-fixing.

The prosecution will argue Cairns knowingly lied when he said he had never been involved in match-fixing.

McCullum is poised to be one of the key witnesses after he testified to anti-corruption investigators that Cairns had tried to coerce him into fixing. McCullum said he was as committed as ever to eradicating corruption from the game.

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"As an international cricketer, you know the rules and you have to follow them. I'm prepared to do my duty," he told Tony Veitch on NewstalkZB today.

McCullum said his was just a small part in the context of the overall trial, yet he has already been a major focus. He recently took out an injunction to prevent the publication of personal emails between him and former mental skills coach Kerry Schwalger.

That saw him become the focus of several outlandish rumours.

"It's gone down the the route of character assassination and it's disappointing that I feel like I'm the one on trial," he said. "There are people trying to serve their own agendas."