The fog in Sydney has delayed many flights.
As a micro example, B738 VH-VXJ has had an eventful couple of days.
Yesterday (Monday 4 August) it operated QF943, the evening 2025 hours BNE to PER airborne at 2146. However it diverted to MEL: I'm unsure what happened to passengers. It then today ran the 0720 hours MEL-SYD QF412 that wasn't in the sky until 0946 hours, arriving SYD at 1042 hours, 117 minutes late. Its next flight is QF922, the 0945 hours SYD up to CNS that's expected to push back at 1120 late this morning. Expect a greater delay due to congestion with arrivals as always taking precedence.
QF's new 'baby' A321 VH-OGA is operating empty ('ferry flight') as QF6104 from MEL to SYD, an 0940 hours timetabled departure that took off at 1009 hours. Arrival at gate (assuming it's going to a gate: perhaps to a hanger?) is forecast for about 1127 hours.