Gregan was part of the last Wallabies team to hold the Bledisloe Cup in 2003 and won 12 tests against the All Blacks.
Wallabies coach Michael Cheika had high praise of the former halfback who famously called out 'four more years' to the All Blacks players late in the 2003 Rugby World Cup semifinal victory.
"There's no one better - he's the number one around his attitude, leadership, work ethic, he's got all those things," Cheika said.
"It's just experience from a player who's been there, done that, well-respected by guys he played against and guy he's played with," he said.
"I played with him and against him and he's so well-respected so I think it was logical for him to come in and I'll try and get that happening on an ongoing basis."