However, Castle has asserted that Rennie is "absolutely committed" to joining the Wallabies once his commitments with Scottish PRO14 club Glasgow Warriors come to an end in June.
"Dave is absolutely committed to Australia and I am absolutely committed to seeing the business through these difficult times and ensuring that we can see the Wallabies back playing test matches with the new coaching team that we worked really hard to put together," she said, according to the Sydney Morning Herald.
"I know that Dave and [assistants Scott Wisemantel and Matt Taylor] are working really hard doing the groundwork to make sure the Wallabies are going to be as prepared as they can be and we have a world-class practitioner in John Pryor leading our athletic development programme. I'm really excited to see that coaching team coach the Wallabies."
Castle's confidence comes in stark contrast to sources reportedly close to Rennie, though, who have indicated to the SMH that the two-time Super Rugby-winning coach would be the first to leave if there is a change in power at RA.
Those sources were also unsure whether the likes of Wisemantel and Taylor would last if Rennie were to depart, given the two assistants left high-paying jobs in England and Scotland for Wallabies roles.
The SMH reports that should any tumult unfold, Brumbies head coach Dan McKellar would be the likely beneficiary if Castle, Rennie and anyone else were to leave RA.
This story was originally published on RugbyPass.com and was re-published here with permission.