All Blacks coach Steve Hansen has heaped praise on Richie McCaw by stating his former skipper is mentally the toughest player he's ever coached.

McCaw played 148 tests for the All Blacks, a record in world rugby, and is the only player to lift the William Webb Ellis trophy on multiple occasions.

Hansen was by his side for large parts of the flanker's career after first bringing McCaw to Canterbury after he impressed as a schoolboy for Otago Boys' High School and then when Hansen joined the All Blacks coaching staff in 2004.

Hansen is heading into his eighth and final season as the All Blacks head coach. The side have played 99 tests with Hansen in charge and have compiled 85 wins and just eight defeats over that time with a staggering win percentage of 90.10.

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After coaching hundreds of players over his coaching career, Hansen said it was McCaw's 'sheer desire' that stood out.

"He's a different cat to most people, Richie," Hansen told Newstalk ZB's Martin Devlin yesterday.

"Mentally, he's probably the toughest player I've had anything to do with, and just a real sheer desire to be better every day. Some people [you have to teach that], but he had it naturally, and it was always a pleasure having him in the team," Hansen added.

After retiring from rugby following the 2015 World Cup, McCaw has proven his mental toughness in developing a passion for multisport and adventure racing. He recently compelted the Coast to Coast two-day event.

"It's just a fun thing to do," he told the Herald this month.

"But it's a challenge I think, as much mental as physical and that's the sort of stuff I quite enjoy."