The ACT, Queensland, Victoria, Rugby WA, Rugby South Australia and Tasmania sent a statement on Wednesday night backing chairman Paul McLean.

NSW, however, was a notable omission from the letter, which pleaded for unity and backed Rugby Australia's "process of transition and renewal".

The code has lurched from turmoil to turmoil in recent times, chief executive Raelene Castle quitting two weeks ago and now Peter Wiggs quitting the board in protest.

Clarke will step in as an interim leader, while McLean attempts to clean up a rugby mess on and off the field.

"As the chairs of Australian rugby's provincial representative bodies and as constitutional members of Rugby Australia (RA), we are fully supportive of the RA board's ongoing work around reform and renewal, which has been underway for a number of months," the unions said in a joint statement.

"Australian Rugby's provincial representatives do not support individuals or groups designating appointments. We are supportive of the process of re-structure with appropriate consultation.

"We feel it is vitally important for all rugby's stakeholders to maintain a calm focus, and to give Paul the support and time he and the Board require, as we collectively navigate through this transitional period.

"We place our trust in Paul and his RA board to continue to consult with his members in the knowledge that there are highly skilled individuals on the RA board, who each possess the experience and acumen that will allow Australian rugby to move forward as we all plan for a gradual return to community and elite rugby in Australia in the weeks and months to come."

Clarke will hold the fort as Castle's short-term replacement as Rugby Australia CEO, his arrival capping a turbulent day for the code that began with the messy exit of board memberWiggs.

Former Brumbies and Rebels boss Clarke will begin immediately and take pressure off interim chairman McLean, who had been balancing both roles since Castle stood down almost two weeks ago.

Clarke enjoyed two stints as Rugby Australia's chief operating officer but won't be auditioning for the role full-time.

"The game is facing some unprecedented challenges and like all rugby supporters and people working within the game, I want to see Australian rugby get through this and emerge on the other side with greater certainty and a positive outlook," he said.

"That is a challenge that I am very willing to take on and that's what the focus will be over the next few months while the board conducts its search for a permanent CEO."

His appointment came just hours after Wiggs' resignation following a disagreement with McLean over the CEO recruitment process.

Touted as RA's next chairman since his arrival in March, Wiggs had been pushing for Australian Olympic Committee boss Matt Carroll to immediately succeed Castle.

McLean entertained the prospect, according to emails published by The Australian on Wednesday.

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However, citing a conflict of interest because of Wiggs' friendship with Carroll, McLean suggested the former remain as a director and instead promote incoming board member Hamish McLennan to the position of chairman.

Fellow board members had reportedly pushed back on Wiggs' hasty plans that would have skipped any formal recruitment or interview process.

Supercars chairman Wiggs, who was instrumental in steering RA's player pay-cut deal during his brief rugby posting, replied by saying what little regard he had for the board had "extinguished" and he would resign on Wednesday.

McLennan, a former Network Ten boss and News Corp senior executive, now appears a frontrunner to assume the chairmanship alongside Clarke.

Clarke has plenty on his plate, with the cash-strapped RA navigating a way back to the field while trying to negotiate a new broadcast deal.

A return to play committee is behind the code's push to launch a revamped domestic Super Rugby season in July, while a Bledisloe Cup series remains a prospect later this year.

RA, which is awaiting rescue funds from World Rugby, faces a potential revenue hit of $120 million if no play is possible in 2020.

"Our absolute priority is to get the game back up and running across the country and supporting our community so that the game is in the best position to move forward from this current period of uncertainty in a position of good health," McLean said.

"We believe Rob is the right person to enable us to achieve those objectives."