If, as you say, both the lodging AND receiving postcodes are within the Australia Post defined express network then Australia Post guarantee next business day delivery.

If this did not happen then Australia Post will refund your express postage charge to you. 

I have eactly this situation with them at present for a 500-gram express satchel that should have been delivered next business day but was not.

In order to lodge a claim you need to log into your Aust Post My Support page and open up a case with them. They will verify the details and either agree to refund or explain why it was not delivered in time.

In my case they have agreed to send out a replacement satchel but you can choose to receive a money order which you can redeem at any Aust Post outlet.