Sonny Bill Williams is a "freak of nature" rated by former New Zealand coach Steve Hansen as the best athlete he has ever coached.

Cross-code star Williams is set to mark his return to Australian rugby league for the Sydney Roosters this weekend, having been left without a team play for after Toronto Wolfpack withdrew from the Super League.

The 35-year-old left the Roosters and rugby league in 2014 but will make a warmly welcomed comeback against the Canberra Raiders on Saturday after undergoing a coronavirus quarantine period.

Two-time rugby union World Cup-winner Williams is a man of many talents, having also had seven professional boxing matches, and for Hansen – who has coached some of union's all-time greats, including legendary All Black Richie McCaw – he is the pick of the bunch in terms of all-round athleticism. 

"The greatest rugby player I've coached was McCaw but Sonny would be the best athlete I've coached from a pure athlete sense," Hansen told the Sydney Daily Telegraph. 

"He's a freak of nature."

Hansen, who stepped down from the All Blacks after the 2019 World Cup, also praised Williams' influence on the younger generation.

"As he has got older, he has matured," added Hansen. "His training habits and his off-field habits are world-class now. 

"He will be a good role model for the young fellas in the club."

Ian Foster is now in charge of New Zealand and Hansen, who admits it will be difficult to watch the side play without being directly involved, expects him to do well.

"The All Blacks haven't had a game so I've been quite lucky to have the opportunity to get used to not coaching them without having to watch them," he said.

"I'm sure there will be a tug in the old heart when they finally get out onto the track.

"But Fossie will do a great job with them I'm sure, and I'm really excited for him to take the team to the next level."