According to Lifehacker, you need to pay the $9.99 monthly fee just to be able to use your monthly data allowance via tethering. There are also extra data packs that you can purchase on top of your data pack specifically for tethering, like 2GB for $25 or 6GB for $45, but it looks like those figures are still on top of the $10 a month you need to pay just for the service. Excess data will be charged at $0.35 per MB. It won’t be available for pre-paid customers to start with, but that will change at a later date apparently.
The tethering service will become available on June 22. We’re seeing that some Optus customers are getting the service now for nothing, but I’d expect that to change on Sunday.