MacDs internet connection is pretty abysmal every time I have tried to use it. Ok for a quick email check, but any thing else is too pain ful to use.
I woudl stick to the telstra mobile plan you have, and use the phone for the internet connection. Telstra plans are garbage, their main strength is the coverage Telstra have out Black Stump way. Also ask any locals or other bush trekkers if theu know of any 'telephone hills' on your way.
As you wrote, you are paying for the data part of your plan any way, and 1.5Gb seems to fit your needs OK.
Many caravan parks and motels have inetrnet connection available for a few dollars ( on long trips, a motel/caravan park for a night or two is a good thng for the relations ship - long , hot shower, no cooking/cleaning for your selves etc) that for a big 'memory dump' are the way to go. Get yourself a 'cloud' photosaving account ( hp and canon include one with their printers and cameras) as a backup, just wind the privacy settings to most restrictive you can.