All Blacks legend Sir John Kirwan has urged Kalyn Ponga not to sign a new contract with Newcastle if he wants to play for New Zealand at the 2023 Rugby World Cup.

The NRL star has reportedly been offered a four-year $4.5-million contract upgrade, which includes an option for him to leave in 2023 to fulfil his dream of playing for the All Blacks.

Newcastle are eager to lock down the 21-year-old who comes off contract at the end of next year and becomes a free agent to NRL clubs on November 1.

Kirwan says the Knights fullback will require at least two years in union and would be taking a huge risk by switching codes just a year out from the tournament in France.

“I don’t think he could do it in one year,” Kirwan, who made the switch from rugby league to union in 1994 when he signed for the Warriors, told the Sydney Morning Herald.

“It would be high risk on his part. He could take that risk if he wants, but this All Blacks team has some guys who are pretty big competitors and like to succeed. I think he needs two years.

“He’s got nothing to lose. A two-year contract and if it doesn’t work out he can go back to league.”

Kirwan, who scored 35 tries in 63 Tests for New Zealand, says Ponga is looking for a different challenge.

“This has nothing to do with money. For guys at this level, what I’ve realised is that they want a different challenge.

“I believe that’s what Kalyn wants, and if he wants to have a crack then it won’t be about money.”