Anon, it doesn't need to be an NBL rule. It's a FIBA rule and I'll stand corrected but the principle is that unless a specific NBL rule overrides a FIBA rule, the FIBA rule stands. The NBL don't restate the three-second, travel or goal-tending rules and similarly now, it's the same in this situation.
Article 46.12: (The referee shall .....) Be authorised to approve and use the technical equipment, if available, to decide,before he signs the scoresheet, if a last shot for a field goal at the end of each period or each extra period was released during playing time and/or whether that
shot for a field goal counts for two (2) or three (3) points.
They made a correct decision. Very exciting finish.