In my experience, its first come, first served for non F customers (who can reserve appointments) for the spa - just ask as soon as you enter the lounge if there is a spot available. Sydney tends to be tight for walk-up appointments; Melbourne much better availability.
I've never used the showers in the F lounge but I doubt there will be a problem getting to use them. For most pax there, the Flounge is prior to their first flight of the day. Again, ask at the front desk on entry how they are looking for use.
