If you're going anywhere rural you have to use Telstra or their budget subsiday Boost. They have the biggest network and best coverage outside the populated areas. That said no one covers the actual outback.
Boost Prepaid 28day plans the lowest tier here has 45GB of data and unlimited calls to the United Kingdom. They have list of retailers where you can buy in store, or you can order them online. I guess you could order them to the UK but it'll sting if they don't arrive before you leave. I usually just get one sim and let my girlfriend hotspot when were away.
Hopefully that helps!