The $9.99 fee -- which will become available from June 22, according to an Optus press release -- doesn't get you any extra data, but does let you draw on your existing allowance. A 2GB bundle costs $24.99, and a 6GB bundle is $44.99. Excess data is $0.35 per megabyte. It would seem likely the bundles only last 30 days, but there's no way of knowing yet -- Optus has announced the pricing but isn't going to reveal the terms and conditions until the service goes live.
Online reports suggest that iPhone users on Virgin have been able to tether, despite Virgin using the Optus network, so it would seem to be a decision based on carrier profit rather than technical requirements. One thing's clear though: if tethering is an important feature for you, literally any other carrier is going to be better value.