New Zealand head coach Ian Foster insisted Wednesday he had known for a month that predecessor Steve Hansen had agreed to help rivals Australia prepare for the Rugby World Cup.

Hansen is assisting Australia boss Eddie Jones, a long-time friend, as an unpaid adviser this week while the Wallabies prepare to face World Cup hosts France in a warm-up match in Paris on Saturday.

New Zealand hooker Dane Coles said he was "gobsmacked" by 2015 World Cup-winning coach Hansen's move.

But Foster, like Coles speaking in London, where the All Blacks are preparing for their warm-up match against reigning world champions South Africa at Twickenham on Friday, was in jovial mood when asked for his views on the subject.

"Outstanding," said Foster. "He (Hansen) has agreed to give me three pages of notes about everything that's going on in their camp... I chuckle at the headlines."

Formerly Hansen's assistant in the New Zealand set-up before succeeding his old boss in 2020, the 58-year-old Foster added: "You guys (the media) must be pretty excited by it, but he told me a month ago he was going in.

"He (Hansen) is good mates with Eddie, but there's no lack of trust (between Hansen and New Zealand) and in his commitment to us. I've got no issues really," Foster also told a news conference after naming his team to play the Springboks.

"In fact, we might bring him in for a couple of days."

Hansen's 'defection' to the Wallabies' camp led New Zealand Prime Minister Chris Hipkins to quip the country should "cancel his citizenship", before adding quickly "that's a joke -- just to be very clear".

In a phone interview with New Zealand radio station Newstalk ZB from Paris, Hansen explained he is observing Australia 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 insisted his involvement with Australia was 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 said.

The All Blacks and Wallabies have been drawn in different World Cup pools but could meet in the knockout stage.

New Zealand play fellow heavyweights France in the opening match on September 8, with Pool A also featuring Italy, Namibia and Uruguay.

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

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