nbn Co performs maintenance to their network whenever they identify a need for it, I'm afraid we can't really control when this happens from Optus, nor are we able to advise exactly what the maintenance works are doing.
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@JenWrenLooks like planned maintenance can be one of the following:
1) Upgrade network
2) Replace equipment with new equipment with extra features or replace obsolete equipment
3) Fix a known issue that is NOT urgent.
4) Something else...
You are probably seeing multiple item 3) over the period of time (Optus uses this item to fix reported problems, aka maintenance to fix issues)