All Blacks coach Steve Hansen says his former assistant Mick Byrne has promised he won't spill the secrets of his time with the team to his new employers the Wallabies, but either way the Australian is looming as a secret weapon for fierce rival Michael Cheika.
Byrne, who was with the All Blacks for 11 years and won two World Cups, has joined Cheika's team after moving back to Brisbane to be closer to his family and help former Wallaby and broadcaster Rod Kafer's Sportstek Academy.
In looking ahead to the Rugby Championship, which starts for the All Blacks on August 20 with a test against Australia in Sydney, Hansen broached the subject of Byrne himself when he said: "It's farcical to think they [Australia] won't turn up and create a tremendous competition in this Championship [despite their Super Rugby form].
"Australia have got four weeks to prepare for the first test, we've got a week by the time we get everyone there, so that's an advantage. They've brought some quality men back from overseas and they've also got a quality coach coming in from New Zealand by the name of Mick Byrne so that will enhance them as well."
When asked about his good mate Byrne, Hansen said: "He's told me he's not going to talk about the All Blacks. He's a good man, I trust his character and wish him all the best and we'll have a bit of fun when we catch up."
Even should Byrne not divulge the secrets of the All Blacks, he will likely play a significant part in improving the Wallabies.
In an interview with Fox Sports in March, before he got the job with Australia, Byrne spoke in depth about how he changed the All Blacks' skill levels when he arrived in 2005.
He made such an impression there that later Byrne, who had a background in the AFL, was promoted to forwards coach.
"Players knew what their poor habits were [in terms of tackling or passing]," he said. "If it was their feet, if it was their shoulders and we worked on it every day. We didn't have a window of skill, we did it every day.
"It's the tight-five who have been the biggest improvers because all through their life it's just been catch and carries," Byrne said.
"If you've got another five players who can carry or distribute, that's another five opportunities you have to move the ball to another area of the field.
"The first thing we did was work on our ability to catch the ball in both hands.
"I know that sounds basic, but a lot of those tight forwards would run on to the ball and take it on their chests because that's the safe way to catch.
"But it means if you take it on the chest you're into contact, so you want to move the ball, take it in your hands and then you can move the ball."
The All Blacks, including tight forwards such as Brodie Retallick, Charlie Faumuina, Dane Coles and Joe Moody, amazed a global audience with their skills during the World Cup and Byrne can take a lot of the credit for that.
South Africa are showing quicker improvements there than the Australians, and particularly the Lions, who will arrive in Wellington for the Super Rugby final against the Hurricanes as outsiders but not without a chance.
Hansen said: "The style of the game they're playing is invigorating and they're spot on with their option taking. They're running from their own goal-line half the time and the Hurricanes will have to be on the job."