The Tokyo-based Sunwolves, currently fourth in the five-team Australian conference, will be chopped from the competition in 2021, when SANZAAR's current broadcast deal expires.

While Castle admitted to concern about the NSW Waratahs, Queensland Reds, Melbourne Rebels and ACT Brumbies playing less Australian derbies when Super Rugby reverts to a 14-team round-robin format, the RA boss said SANZAAR booting out the Sunwolves was all about the bottom line.

"The reality is that, as much as we see Japan being an incredibly important part of the future of rugby, when the JRFU withdrew their underlying support for the Sunwolves and they couldn't find another way to guarantee that underwrite, it left the SANZAAR partners in an exposed positions financially," Castle said after Friday's announcement.

"And we didn't think that was in the best interests of the SANZAAR joint venture partnerships."

Castle said the money required to prop up the Sunwolves would effectively come out of the mouths of the other 14 teams in the competition, including Australia's four.

"That's at the top line and then there's also the travel component now as well," she said.

"When you have a competition that has 14 rounds and reduced with the Japan element, (it) also gives you some savings on the operational sides of the business."

The Sunwolves, who have won just seven of their 43 matches since joining the competition in 2016, avoided the chop when the Western Force, along with South Africa's Southern Kings and Cheetahs, were cut after last year's competition.

Reverting to a 14-team round-robin format will see the removal of the current three-conference system that has proven unpopular with fans and broadcasters.

But it also means each team will only play 13 games during the home-and-a-way campaign, instead of 16 they play in the conference format.

The Waratahs, Reds, Brumbies and Rebels will only play three Australian derbies every two seasons from 2021 onwards, as opposed to three a year under the current system.

"Yes, there's concern but that concern is shared by Rugby Australia," Castle said.

"That was part of the modelling that we did to make sure we took those things into account when this decision was made.

"Do we believe that this structure gives us the best platform to negotiate a good outcome with our broadcasters and commercial partners? Yes we do."

In addition to Australia's four teams, Super Rugby will contrinue to comprise of the existing five New Zealand sides, four from South Africa and the Jaguares from Argentina.

Australian Associated Press