Charles Piutau explains his decision at a press conference in Auckland © Getty Images

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Charles Piutau understands he has put at risk All Blacks Rugby World Cup selection after signing a two-year contract with Irish club Ulster reportedly worth NZ$2 million.

Piutau, who will start his two-year contract with the Belfast club in July 2016, described his decision to head to Northern Ireland as "tough".

"I can assure you I've been through an intense past few weeks, consulting with friends, family, church, and other various people, who are part of my support team, to make this decision," the Blues and New Zealand All Blacks fullback/winger said in confirming his decision.

"I'm looking forward to a new challenge of living in a new country, experiencing a new culture and playing a different style of rugby. It's a two-year plan which I believe will develop me as a player and as a person.

"My firm focus right now is the Blues and my team-mates, to work hard at turning around our season. We have a tough game this week against the Chiefs, and I'm also passionate about playing for the All Blacks. I'll do my best and put my best foot forward to wear the black jersey at the end of the year."

Charles Piutau had established himself as a key member of the All Blacks squad © Getty Images

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All Blacks coach Steve Hansen is "extremely disappointed" that Piutau is leaving New Zealand, saying "he was a person we were hoping would grow over time after this World Cup in to one of our senior players".

"We've put a lot of time and effort into him so we're disappointed he's decided to go down that route."

Charles Piutau is one of the rising stars of New Zealand rugby © Getty Images

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Hansen said that Piutau's decision certainly did not make him ineligible for continued All Blacks selection and World Cup consideration, "but it does make it harder for him".

"He's going to have to play really, really well. We've got a duty to pick the best team but at the same time if two guys are vying for one spot then things like commitment and experience come into it. Not only are you picking for today but you're also picking for tomorrow. That's what we've done with him for three years."

Piutau said "all I can do is focus on my performance and I'm going to be playing out of my skin for my Blues team this season". He also said "I don't think I can find words to express how hard it was" to rule himself out of contention for the 2016 Olympic Sevens team and the 2017 British & Irish Lions tour of New Zealand.

Piutau is understood to have signed a two-year contract with New Zealand Rugby after two months of negotiations, but the New Zealand Herald reported the contract "stayed in the offices of Piutau's management team as the day before he signed it, an unsolicited offer arrived from Ulster".

"Piutau was asked, after he had signed but not sent his contract to the New Zealand Rugby, whether he would like to explore the Irish offer," the New Zealand Herald reported. "He said yes."

Piutau then signed the Ulster and "asked his management team to rip up the signed offer from New Zealand Rugby".

He will play overseas elsewhere before taking up his Ulster contract if he is not able to play for the Blues.