At that time of the night there’s practically no one around. There’s usually a lot of track shortening given to try and accommodate you before curfew. The reason going lower is that there’s thicker air. This will give us a higher True Airspeed and allows us to fly faster (albeit burn A LOT more fuel). If the tailwind is strong enough then going higher might have given you more of an advantage than flying lower with a lower tailwind (usually the prevailing wind between MEL and SYD).
In terms of flight planning, if our ETA is 20mins before curfew an alternate will be provided regardless.
