All Blacks great Christian Cullen warned lucrative sabbaticals abroad would "kill" rugby in New Zealand and turn the players into "mercenaries".

Cullen, speaking on the sidelines of this weekend's Hong Kong Sevens, said New Zealand's decision to allow star flyhalf Dan Carter a six-month stint in France set an unwelcome precedent.

"I think it's dangerous. You give Dan Carter that, you have to give Richie McCaw the same opportunity, Ali Williams, Mils (Muliaina). I think it's setting a dangerous precedent," Cullen said.

"You don't want the All Blacks to become mercenaries where all the good All Blacks are overseas and then they come back and play Test matches, because it will kill the game in New Zealand."

No overseas-based player is considered eligible for the All Blacks but New Zealand let Carter sign a six-month 'sabbatical' contract with Perpignan worth a reported 35,000 euros ($A67,460) a match.

The controversial move was further clouded when Carter suffered a serious ankle injury after just five games, leaving him sidelined for at least six months.

"Personally I think it's a dangerous move. It's good for the players because they can go and make money and, as we all know, rugby doesn't last forever," said Cullen.

"But on the supporters' side it's dangerous because the public want to watch the best players in the world. New Zealand could slowly become mercenaries - the overseas All Blacks come back and play Test matches and they're gone.

"And our domestic competition could be wiped out."

Cullen, an inspirational fullback who scored a then record 46 tries in 58 All Blacks appearances, said the domestic NPC tournament was already suffering from an exodus of players after the 2007 World Cup.

"Already the NPC is killed in New Zealand," he said.

"In the past you could see five, six, seven All Blacks playing against each other in one game. Now, that's not the case."

Cullen said players should be forced to choose between playing for their country or for a foreign club.

"I just don't like the idea. I'm an old sort of traditionalist player. If you play for the All Blacks, you play for the jersey, if you go overseas you play for that team," he said.

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