My suggestion is to go for 40 to 50 Mbps plan as that's sufficient for watching Netflix movies and YouTube videos simultaneously on two devices. If you are a family of 3 i.e. two adult and 1 kid, 45 Mbps speed with unlimited data is good enough for your needs even if you watch videos whole day on multiple devices. The NBN provider will try to sell you the 100 Mbps plan but you don't really need it. In case you do, you can always upgrade your plan later.