As I understand it, SYD ATC is stretched for staffing resources. They ROSTER "understaffing" on Sunday to be able to use their scant staff on busier weekdays. One position they might have do do away with for periods is FLOW. When there are weather diversions the predictability of landing times is more difficult. if there is no FLOW to roll with the punches and keep optimising the sequence as a result of diversions they have to utilize a fairly loose arrival gap to give approach a fighting chance to deal with what they are dealt. Inevitably there are suddenly opportunities for delayed aircraft to fill gaps because others divert and are late for their assigned landing times. This is when we might get "Increase speed, cancel speed below Ten" etc after a holding delay. The major issue is understaffing, and overtime fatigue for those that are there and who are fed up with being called in to work on days-off to prop up the system.