The Australian Energy Market Operator (AEMO) controls the National Electricity Market (NEM) in Australia, by ensuring that supply and demand are always in sync.
However, on rare occassions demand (energy consumption) may exceed supply (energy generation) in certain areas of the NEM (states, cities, suburbs).
These rare events are usually referred to as a Lack Of Reserve (LOR).
AEMO has a few options to respond to these LOR events, and one of them is "Load Shedding".
Here's AEMO's explanation:
"Load shedding is the controlled reduction of electricity supply to parts of the power system servicing homes and businesses to protect system security and mitigate damage to infrastructure."
In simple terms, this usually means "rolling blackouts" coordinated across different areas where there isn't enough electricity supply.
These Load Shedding events are mandated by AEMO, and enacted by Distribution Network Service Providers (DNSPs). In Victoria, this could include Jemena, AusNet Services, United Energy, Citipower, and Powercor.
For customers, it would be like an unplanned outage, often with little warning, and they can potentially last several hours.
Whilst these events are rare, they can still happen at anytime when the energy system is out of balance. They're more common in summer, like during a heatwave, but they can also occur at other times of the year.
For more information, take a look at AEMO's Load Shedding Fact Sheet.