Oh, so it probably slipped through the cracks. On a 737, unlikely you wouldn't get any seat in rows 4/5 as a P1 prior to T-80 at that time.Not sure how far ahead it was booked. Too many flights! Typically a couple of weeks. You can't select seats on Dash 8.
Even on SYD-MEL during peak hour. You would have to have 12 P1/CLs on that flight who beat you to it for that to occur. Not impossible but not overly likely either.