Veteran All Blacks hooker Dane Coles said he was "gobsmacked" to hear former New Zealand coach Steve Hansen is helping rivals Australia prepare for the Rugby World Cup.

Hansen has agreed to assist Australia head coach Eddie Jones, a long-time friend, this week as an unpaid adviser while the Wallabies prepare to face France on Sunday in a World Cup warm-up played in Paris.

Coles, who has played 86 tests for New Zealand, was stunned to hear his ex-All Blacks boss is currently in the Wallabies camp.

"Yeah? Shag what are you up to," the visibly shocked Coles said, using Hansen's nickname.

"That hurts a little bit, to be fair," he added.

"I'm actually gobsmacked. He's a bit of an icon in the All Blacks set up. I'm a bit speechless. It's a bit disappointing, but we can't do much about that."

New Zealand reporters broke the news to Coles following a training session in London, where the All Blacks are preparing for their own World Cup warm-up against reigning world champions South Africa.

Hansen coached New Zealand to their 2015 World Cup triumph, in which Coles started in the final against Australia.

"He's a great man. He's obviously not in our environment at the moment. I suppose he can help out Eddie," Coles added.

"Hopefully, he doesn't tell Eddie all our secrets. I think he'll be respectful, that's the main thing."

Loose forward Ardie Savea was more circumspect, saying: "Everyone's got to do what they have got to do. Good on him."

In a phone interview with New Zealand radio station Newstalk ZB from Paris, Hansen explained that he is observing the Wallabies this week as a favour to Jones.

Hansen, 64, said he would "just like to put everybody's mind at rest that I haven't joined the Wallabies for the Rugby World Cup."

"I'm only here for about three or four days at the request of Eddie, a good mate of mine, just to give him some feedback on what he is doing. Rugby is bigger than all of us, so happy to do that," he added.

Hansen made it clear that he is assisting on a voluntary basis.

"If you are working for someone, you are getting paid. I'm not working for anyone, I'm just here as a friend," he added.

The All Blacks and Wallabies have been drawn in different World Cup pools but could meet in the knock-out stages.

New Zealand play hosts France in the opening match on September 8 – the two heavyweights drawn in Pool A alongside Italy, Namibia and Uruguay.

Australia play Wales, Fiji, Georgia and Portugal in Pool C.