Holmes shot to prominence late last year when Wallabies coach John Connolly selected the 19-year-old to be part of his training squad for Australia's northern hemisphere tour in November.
Although Holmes didn't make the eventual touring squad, it marked him as a player of the future despite his lack of Super 14 experience.
While the Brumbies will officially make the announcement of Holmes' signing next week, Brumbies coach Laurie Fisher and Waratahs boss Ewen McKenzie made it clear that Holmes will take over from Gregan next season.
"We believe he is the best young half-back in Australian rugby," Fisher told The Canberra Times.
"He's quite tall, he's a large player, he's got a good running game. He's a good communicator on the field so we think he's going to be real value add for us."
McKenzie, meanwhile, revealed the Waratahs had made 'overtures' to keep Holmes at New South Wales but all but confirmed that Holmes will be leaving.
"We have to be in a realistic frame of mind in professional sport that players will have to look at their options and work out what is best for them," McKenzie said.Â
"The reality for him is he is an interesting commodity and there are a few teams out there, namely the Brumbies, who are going to lose George Gregan, so there are obvious holes [to fill] elsewhere so he has to look at his options.
"The other reality is that 50% of the player base of rugby in Australia is from New South Wales, but we only have 25% of the contracts, so logic will tell you that 75% of the contracts are outside of New South Wales.
"So people are inevitably going to leave to seek an opportunity somewhere else - that happens occasionally - we do our best to keep players but we can't keep them all."
Ironically, the dynamic young number nine will make his belated debut in the Super 14 this weekend for the Waratahs against the Brumbies on Sunday.
Meanwhile, Brumbies officials have said they are close to agreeing terms for Wallabies trio Mark Gerrard, Clyde Rathbone and Mark Chisholm.