Most phones give you the option to turn access to data while roaming on and off, allowing you to control when your phone is able to access data while you're overseas.
Why is this important?
Unless you're using $5 Roaming, your roaming data charges are higher than within Australia, and excluded from monthly billed plans. So they can add up pretty quickly.
Many phones will use data in the background without you even noticing. So even if you're not planning on actively using data, it's a good idea to make sure it's switched off if you're worried about costs.
Similarly if you’re using Wi-Fi overseas and the signal drops, your phone can switch back to a mobile network without you noticing. Turning data roaming off stops this from happening.
Most phones disable roaming data by default. So if you do want to use it, you'll often need to go in and switch it on first.