Fox Sports has televised Super Rugby and the major southern hemisphere national team tournaments since their inception in 1996 but another party, believed to be Optus, has also reportedly expressed interest.

The current five-year deal expires at the end of 2021.

'We believe that we have more than one party that's prepared to step forward and have a serious look at the rights for rugby,' Castle said at the Super Rugby season launch on Thursday.

Castle was adamant RA's choice wouldn't just come down to the highest bidder, revealing three key criteria would govern their decision.

"One, the number is obviously crucially important, that's how all sports survive in the professional era," Castle said.

"The second thing is access. How can we expose Super Rugby more readily to more people more often?

"There's no doubt the combination that's proven to be successful in NRL and AFL is a combination of some free-to-air access and some behind the paywall access.

"It's something we think will help us grow the numbers and grow the brand.

"Third is the commercial support across the whole sport that broadcasters are prepared to engage in.

"Those are the the main factors but when you get into the detail the list of requirements is probably 400 things long, so it's much more complex than people realise."

While Super Rugby's conference format has come under fire in recent seasons, Castle pointed out the viewership figures had improved last year.

The competition reverts to a round robin format in 2021.

Australia have found it difficult to qualify a team for the playoffs other than their conference winner in recent seasons, but Castle said she could see no reason why they couldn't get two teams through this year.

Meanwhile, the Waratahs have signed Tongan-born Japanese Sevens international Siosifa Lisala on a one-year deal

The centre joins heads to Sydney from Japanese second division club Toyota, where he was developed by new Waratahs coach Rob Penney.

Australian Associated Press