If you turn International Roaming Off, then you won't be able to use your phone overseas.
International Day Pass is usually automatically active on newer plans, but if you log in to your account and check, then that would be a good idea.
You only get charged for the International Day Pass on Days (Eastern Australia Time) that you use the mobile network overseas. If you make or receive a call, SMS or use data, you will be charged for that day. If you turn your phone off and don't use it for a few days, you won't be charged for those days.
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.