3G sunsetting means that a mobile network operator (or carriers) shuts off the cellular infrastructure required to operate communication devices based on 3G (UMTS, HSPA, EVDO) technology. These 3G networks are slower and less efficient than newer 4G and 5G networks, which share the same spectrum.

Australia’s big three, Telstra, Optus, and Vodafone have set different dates for the official discontinuation of 3G support. 

Telstra plans to complete the 3G shutdown on June 30, 2024, marking the end of 3G services for its customers.

Optus is scheduled to stop supporting 3G in September 2024, thereby discontinuing 3G services for its users.

Vodafone is ready to cease its 3G support in December 2023, as it transitions its customers to newer network technologies.

Yes — devices based on 2G and 3G devices will stop working when these networks shut down, and some will lose functionality prior to those network sunset dates, due to a number of factors, including carrier resource allocation.
 
And if you have a device with 4G LTE that supports 2G/3G fallback, make sure that it has the latest device and modem firmware running. Because at the time these devices and modems were built, it was assumed that 2G and 3G networks were always there, in addition to 4G LTE. Some older firmware cannot handle the absence of 2G or 3G networks properly, which may result these devices to be stranded and not being able to connect to the 4G LTE network.
 
To continue their migration to new networks, mobile network operators simply cannot support all older network technologies. Putting resources into 4G and 5G networks is a business necessity.