Im joining the Australian army reserve but they want me to go to a unit 1st and then do basic training later?

You have to do a basic nav and med course before you go to 'Pooka. Depending on your area of service you MIGHT get lucky and be sent to IET before basic to streamline your enlistment, but that's a rarity. I hear the waiting list for 'Pooka is 8+ months right now.