A departure burst of long-haul international flights tends to occur in the last two hours before the Sydney Airport curfew begins. 

Heavy jets, especially those flying long-haul routes, require the use of the westernmost parallel runway (Runway 34 Left) because it is the longest runway.  The extra length is needed for a safe take-off.  For this reason residents in suburbs under flight paths from this runway are likely to notice increased movements in the hour before the curfew. 

You may notice that these heavily laden jets tend to climb more slowly than lighter aircraft.