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NBN-FTTC is one of the better NBN technologies, it has fibre optic to a pit close to your house/unit.
Most 50/20 plan users with UN-COMPLICATED internal cabling will nearly always hit that 50mbps speed.
100mbps plans are a lot more variable, mostly due to internal house wiring/copper leadin cable.
If you dont get an acceptable speed close to the 100mbps, TPG will let you shift down to the 50mbps plan.