All Blacks great Zinzan Brooke has lauded the career achievements of Sonny Bill Williams – but believes his switch to sevens for the Olympic Games could lead to a disastrous campaign for New Zealand in Rio.

Williams will feature for the men from the land of the long white cloud in Rio – Gordon Tietjens’ men kick off their bid for Olympic glory against Japan on Tuesday – but Brooke has reservations about how much of a success the 31-year-old convert will be.

“Sonny has played rugby’s both codes, he’s tried boxing, he’s won two Rugby World Cups, his CV is very impressive, but I have my reservations,” Brooke told Sport360 on a visit to the UAE earlier this year.

“I should be backing and encouraging him but I’m not too sure he can do it. I hope he proves me wrong. I’m concerned about the New Zealand side.

“No doubt he can find a gap but in sevens there’s many scenarios where you have to get up when you can’t and there’ll be many of those, particularly against the Fijians.”

The 51-year-old former No8 won 51 All Blacks caps between 1987-97 and was renowned himself for being an athlete who broke barriers.

Brooke scored a then record for a forward 17 Test tries on the international stage and famously kicked a 45-metre drop-goal against England in New Zealand’s 45-29 semi-final win against England at the 1995 World Cup.

Brooke doesn’t feel his nation have enough to challenge Fiji in Rio, with Ben Ryan’s men favourites for the gold medal.

The once-dominant Kiwis are record 12-time winners of the World Sevens Series, which has run for just 17 years.

They are also joint holders of the best record at the Rugby World Cup Sevens along with Fiji, winning in 2001 and 2013, but Brooke claims Fiji are in a class of their own in Brazil.

“Ben Ryan has them in another stratosphere and they’re the team to chase,” said Brooke.

“I don’t think New Zealand sevens has enough to challenge Fiji. I think they need new blood but it’s too late to change to win the Olympics. You can’t blood someone in that late (Williams) so it potentially could be a catastrophe short and long term.”

Aside from the Olympics, New Zealand are gearing up for the Rugby Championship which kicks off with Steve Hansen’s men visiting defending champions Australia in Sydney on August 20.

In 20 years of the southern hemisphere competition, the All Blacks have lifted the trophy 13 times, but the Wallabies broke their three-year grip on the title last year.

Argentina have been in the tournament since 2012 and broke new ground last year by finishing third. It was the first time in four editions they had not come bottom of the group and Brooke believes they can beat the wooden spoon again this year.

“The Irleand team in the World Cup was good, but Argentina exposed them badly,” said Brooke (the Pumas defeated Joe Schmidt’s men 43-20 in a thrilling quarter-final encounter).

“It was a hell of a game. What they did to Ireland was put 43 points on them, Ireland who won Six Nations. That says a lot. Argentina are a young team too, you should be watching them. They won’t come last in the Rugby Championship this year.”