The NRL has stepped in to block Danny Buderus from coaching the ACT Brumbies just hours after the Super Rugby club announced the rugby league great would be a consultant coach.

In a bizarre day for the two rival codes, the NRL refused to allow Buderus to become a part-time coach in rugby union while still contracted to play rugby league.

But the Brumbies are waiting for an official word from the NRL and Buderus to say he definitely will not be with the club next season.

The NRL was unaware of the move until after the Brumbies announced his recruitment.

What started as a day of breaking down barriers ended in a cross-code battle which will ramp up the rivalry between the codes.

Buderus, a Newcastle Knights legend and a champion hooker, was unveiled in Canberra as the Brumbies' new ''collision coach'' at noon on Thursday.

But just four hours after wearing a Brumbies training jersey, his hopes of joining Jake White's coaching staff were dashed with the NRL putting the kibosh on the job.

Buderus is a former Australian and NSW State of Origin captain and a premiership winner. The 34-year-old has one year remaining on his playing contract with the Knights and is readying to retire.

While plenty of rugby league players have developed into rugby union coaches, linking with the Brumbies while still playing with the Knights is unprecedented.

But because of salary cap and third-party implications, and because it was concern the move promoted a rival code, the NRL stepped in.

''It is hard to envisage any circumstance where an agreement could be approved that allowed a current NRL player to promote another code competing in the same market as an NRL team,'' interim NRL boss Shane Mattiske said.

''The announcement today which saw Danny at a press conference in another code's training gear underlines potential issues.''

It was a whirlwind six hours at Brumbies headquarters at Griffith. While the deal had not been 100 per cent finalised, the Brumbies were confident the Rugby League Players Association would clear it.

White, the Brumbies head coach, and Buderus fronted the media at noon to talk about how the Knights legend would help the two-time Super Rugby champions be more physical next season. South African White even spoke of his pleasure that rugby league and rugby union could share knowledge and information to improve their respective sports.

Buderus was supposed to train with the Brumbies for the first time on Thursday afternoon, but as the drama unfolded he instead flew back to Newcastle.

Buderus - who has never played rugby union - was determined to make it clear he was ''first and foremost a rugby league player'', but he was excited to join the Brumbies.

''It will be fine,'' Buderus said of the deal before the NRL stepped in. ''I've got things in place, I've got a supportive club in Newcastle. It's not a betrayal [of rugby league], I hope it's not out there with the rugby league and rugby union thing … I don't think there's too much animosity there.

''It's just a coaching thing and hopefully I can benefit these guys. I'm looking forward to the challenge and I'm lucky to be here.''