It does sound like a good deal, but unfortunately, you cannot do that. Welcoming bonuses are offered to new users to boost their confidence and have some cash to continue playing and build their trust in the website. These are not for cashing out without even playing. These bonuses have some wagering requirements that can be found in terms and conditions. You need to unlock bonuses and promotions to be able to cash these out.
The terms and conditions vary from one Australian casino site to the other, and you can find them under their terms. You need to play for some amount of money before you are eligible to withdraw the money. You can always call the respective customer care of an Australian online casino and ask them for more information.