According to that updated policy statement "Table 1. Airspace Criteria Thresholds" it looks like the criteria for a TWR now covers PTO pax numbers, not simply aircraft movements. Maybe I've missed something in recent years.
Based on the numbers, control tower for Ballina?
That'll please the locals. Things won't be quite so free and flexible, with ATC in attendance and obliged to provide separation.
If other places are any indication, it will cost quite a few million $ to put a TWR in ($M10?) and a few million per year to run, and the costs passed onto the users and pax.
Fortunately Clause 22 of the policy statement gives CASA an out.