If you have access to internet (even wifi), you could use their online tool to process a recharge.
https://recharge.boost.com.au/boost-recharge/
If processing it via the phone app, it required mobile data to proceed.
While roaming, that means you need roaming data add-on.
Better to use the link above instead, if you can.
* I am a Boost Australia customer. This is a peer to peer community where users help each other.