If that is your first bill then most likely you got charged prorated for the plan and a month in advance. The month in advance for the plan will be returned back to you at the end of your contract.
It would be best to contact Optus for assistance. They should be able to check the bill for you. Their direct number is 133937. Or you can also contact them via chat, please go to: http://www.optus.com.au/shop/notices/service-chat
Hope it helps.
Just checking in to make sure you've found out why your bill was higher than expected? If not, please send your account details to us via private message and we'll take a look at your bill 🙂