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Rennie was officially confirmed as the next head coach for the side on Wednesday, with his contract taking him through to the 2023 Rugby World Cup.
Speaking to Radio Sport's D'Arcy Waldegrave, Castle said Rugby Australia wanted to be proactive in their recruitment process after Michael Cheika indicated he would not seek another term at the helm if the side did not win this year's World Cup.
"We needed to make sure we had options, or at least understand what the market was telling us about availability; great coaches don't hang around waiting for job offers, you have to go looking for them and make sure that you understand what their contract situations are," Castle said.
"We'd certainly had some conversations with Dave – I'd met with him and then when we exited from the Rugby World Cup and Michael said that he wasn't going to step forward I could continue conversations with Dave."
While Castle met with Rennie initially about half a year ago, Rugby Australia could not engage in contract negotiations until they knew what their coaching situation would be going forward. After a quarterfinal exit, they were able to proceed with their recruitment process and had come to an agreement with Rennie within the following month.
"He was by far and away the most outstanding candidate," Castle said.
"We believed he was the best fit not only from a performance coaching point of view but also a cultural point of view and his experience in bringing groups of people together is very sound.
"Dave's proven ability both at the Chiefs and what he's done in a short time with the Glasgow Warriors to bring together a group of men and build a culture where they all want to work together toward the same thing is one of his best skill sets and that's something we know will be very valuable to us.
"We had a plan and we worked really hard to execute it."