A couple of additional thoughts:

SYD departures sometimes delay to avoid landing in SYD prior to 6am local as SYD has a curfew. Often find they will close the doors a bit late for this reason.
If you are just a few mins late go to the gate for BNE which usually leaves 25/30 mins later. MEL another option. That’s a much better Plan B as you still get to Australia and Virgin can take you down the coast to SYD on the 1 hour domestic flight with lots of frequency. Note - BNE domestic terminal is in a separate building so transfer is a bit of a pain.