Castle, who added that South Africa and New Zealand are both reviewing their rules surrounding overseas-based players, said it would "irresponsible for us not to review it".
The rule has a loophole where players such as Nic White and Matt Toomua are allowed to be picked in the World Cup squad despite not meeting criteria, as the pair have both signed to return to Super Rugby in 2020.
Rugby Australia investigated making a similar deal with lock Will Skelton, but he re-signed with Saracens, instead aiming to return to Australia in 2021.
"He basically had to give up the last year of his Saracens contract, and sign with Rugby Australia and we would have had some flexibility like we did with Nic White and Matt Toomua, they could finish their seasons and come back and play the back few games of Super Rugby, that would have been within the bounds of the Giteau law," Castle said.
"But he was looking to not come back until the following year and that wasn't acceptable to us."
Castle stressed that even if they changed the law, there would still be a rule in place to keep top players in Australia and playing Super Rugby.
"We need our good players playing Super Rugby, so we need to make sure we look at it from that perspective.
"Don't give any open doors for opportunities for players to think 'well I'm going to disappear off and not play Super Rugby and then I'm a dead cert to be selected for the World Cup.
"So that's the bit that we're trying to balance."