If you are flying around Australia, there is only one carrier to choose from: Telstra has the greatest coverage by far. You would need to go very far from civilisation to not get a signal at 3,000 feet!
How Much Data Does OzRunways Use?
Downloading all the maps, ERSA, DAPs and terrain with every update, you would not use much more than about 6 GB per year. Though most people will download these large files while at home on their internet connection, and only use their cellular data for in flight services, which is the most cost effective method!
Things you may be accessing via cellular enroute would be up-to-date weather briefings, restricted area briefings (for the red restricted area shading), and OzRunways traffic. We've optimised all of these to use the least amount of data possible, and at the most you would probably find it to be no more than ~20 megabytes an hour.
So while you are flying and all aeronautical data is downloaded, you will be using little to no data on your plan.
Prepaid or Plan
You can go on any monthly plan, but we find the best value and most flexible to be pre-paid. At the time of writing (May 2022) Telstra has a pre-paid offer of 40 GB for $150 valid for 365 days.
What about using my phone's data?
All phone plans come with cellular data. You can use features such as Hotspot (Apple) or Tethering (Android) to connect your iPad/tablet to the phone, and use the phone's data for an internet connection.
It's can be a bit fiddly setting it up before each flight, but definitely a viable option.
Apple Hotspot instructions: https://support.apple.com/en-au/HT204023
Android Tethering instructions: https://support.google.com/android/answer/9059108?hl=en