Australian Rugby Union boss John O'Neill denies the return of the rugby league defector Sonny Bill Williams will turn into a circus but suspects the New Zealander's presence in the Barbarians team to play the Wallabies will put "bums on seats".

The ARU confirmed on Monday that Williams will play with the Barbarians for the June 6 clash against Australia at Sydney Football Stadium.

The dynamic New Zealander caused a furore last year when he walked out on a contract with the Bulldogs rugby league club to join French union side Toulon.

"I'm looking forward to playing for the Barbarians in Sydney and being on the field with some of the best rugby players in the world," Williams said in a press release issued by the ARU.

O'Neill fended off plenty of questions about Williams at a Monday press conference in Sydney and didn't deny the controversial New Zealander's appearance would generate plenty of publicity.

"Will he put bums on seats, I suspect he will and would I be happy about that, yes, I would," O'Neill said.

"He's taking his rugby career seriously. We weren't involved whatsoever in his defection .. it happened.

"It's been settled legally and he's free to play and we are keen to see a fantastic contest and (if) the fact that he's part of it adds to the lustre."

The former New Zealand rugby league international was the only player confirmed for the fixture, but O'Neill denied the announcement was a publicity stunt aimed at the league State of Origin match to be played the same week.

"I don't believe it is, it's a historical match, the Wallabies versus the Barbarians, the first time we've played them on home soil," O'Neill said.

He was adamant Williams returning to play in a rugby league heartland wasn't rubbing the rival code's nose "in it".

"The fact is that there is a bit of a revolving door these days between rugby league and rugby union."

O'Neill said he understood that announcing the involvement of the notoriously injury-prone Williams a few months before the game was a risk.

And it was up the Barbarians to decide whether to select another recent convert in former Penrith rugby league star Craig Gower, whom O'Neil said was playing exceptionally well for French club Bayonne.

However, former St George NRL star Mark Gasnier wouldn't be available as his club Stade Francais will be in the finals of the top French competition.

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