So Noriega's advice to new Wallabies scrum guru Mario Ledesma is simple: close your eyes and embrace the biggest stage of all.

Ledesma has become the reluctant centrepiece of a World Cup semi-final between the country he played 84 Tests for – Argentina – and his Australian mission to turn the Wallabies into a scrum force.

Ledesma has even asked Wallabies coach Michael Cheika if he's allowed to sing the Argentina national anthem in the Australian coaching box before kick-off at Twickenham on Sunday (Monday AEDT).

Noriega knows what it's like to be split down the middle. He played for Argentina at the 1991 and 1995 World Cups before Rod Macqueen recruited him to the ACT Brumbies and Wallabies.

And what did he do when he was wearing Wallabies gold against Argentina in Buenos Aires in 2002?

"I stood there and I sung the Australian national anthem with my mates," Noriega told Fairfax Media.

"I will never forget that moment, playing for my new country against my old country.

"My situation was different to Mario's, I moved my family to Australia and it is my home.

"I will die as an 'Argie', I will never forget my roots and passion. But it was an honour to represent Australia and enjoy the Australian culture.

"Mario is doing a great job with the Australian scrum. He will be dreading this match but looking forward to it at the same time."

Ledesma, dubbed "Super Mario" because of the magic he has worked with the Wallabies' scrum, wanted to lay low this week to avoid the hype of his country colliding with his job.

Argentina captain Augustin Creevy was Ledesma's understudy in the Pumas side.

Ledesma is rated as one of Argentina's greatest players and led their charge in four World Cup campaigns.

The Pumas have beaten the Wallabies just twice since 1997, and Ledesma played in the shock victory almost 20 years ago.

But now he sits on the other side of the fence and Argentina's players believe his presence has transformed the Wallabies' set piece.

"Mario has brought a change of mindset to Australia's scrum," said prop Marcos Ayerza.

"Instead of being a platform to get the ball in play for them, they now look for psychological domination."

Ledesma has gone from the "video intern" at the NSW Waratahs to a crucial member of the Wallabies coaching staff.

Noriega knows the path well, having represented the Pumas at two World Cups before playing for Australia and the Brumbies and then joining the Wallabies coaching staff after he retired.

Noriega's family still lives in Canberra while he coaches in France. But his Argentinian passion still burns deep.

Noriega even coached Ledesma in 2004. "I remember one game, he was absolutely awesome. But I don't want to be in his shoes this week," Noriega laughed.

"The Australian and Argentina scrum philosophies are different, from the grassroots level. But Mario has done a wonderful job, from my point of view, and I think Australia has one of its best scrums ever."

Noriega was working as a grocery store delivery man in Argentina when Macqueen gave him a call to join the professional era in Australia and it changed his life.

"I'd lose my job every time when I played for Argentina; it was different in those amateur days," Noriega said.

Wallabies 1999 World Cup-winning coach Macqueen said: "Pato [Noriega] had a passion for scrums, he certainly brought that and we're seeing one of the big differences we're seeing now is the scrum [under Ledesma].

"Ledesma was a great player himself. Back then we heard Pato was interested in coming over and he turned out to be a fantastic acquisition."