There won't be relegation from the A League down.
Firstly, because you can't sell someone a franchise and then allow them to be removed from that league. It would tank every clubs value instantly and probably ruin the entire business model overnight.
Secondly, the NSD is running in the off season anyways. So it's for nothing to do with the A League outside of an opportunity to loan out young players.
This competition isn't an actual second division, it's an entirely different competition within the same country. Yanks have the same model over there, MLS which is separate from USL, which is also somehow separate from USL2. No teams get relegated or promoted from any level of "soccer" over there. Looks like we will do the same, can't wait until we start having a Player Draft and everything, Jets will be able to sign the best QB almost every year.