Optus' 100Mbps plan is what they call the "Optus Premium Speed Pack". An extra AU$20/mo on almost any Cable plan, plus a new DOCSIS3.0 modem, and you'll be chewing through your monthly quota about 5 times as fast. Then they throttle you.
If you get the more expensive Fusion plans (the 'Unlimited' ones) the throttling speed is 256kbps. They try getting away with that because 256kbps is the minimum speed to technically be 'Broadband'. ACCC, attack!