GreenPower is a government-led scheme. It creates additional demand for large-scale renewable energy certificates and that should in turn drive up the level of investment in renewable energy generation. You can choose 10%, 50% and 100% of your energy to be tied to these additional certificates. The extra cost is 0.4c, 2.0c, and 3.9c respectively (all per kWh).
We know that's expensive for most people at a time when energy prices need to be lower, so we purchase carbon offset certificates for 100% of the energy that our customers and our own business use. That means that all customers can be doing something positive without paying a premium.