I agree the caption is lacking in detail (for IS editorial criteria) and would include some brief information on what they are protesting against. What exactly is this carbon tax, does it merely tax vehicle emissions or electricity consumption or what does it entail and how would it affect these people? I would also provide more specific location information, such as "Protesters gather in front of the Town Hall in Canberra...". A simple one-liner caption won't usually cut it for IS - especially when half your caption repeats the same information. Basically, caption the images in such a way where someone (like me) living outside of Oz could look at the image, read the caption, and get a basic yet good understanding of what is going on:
Canberra, Australia - March 23, 2011: Thousands of protesters gather near blahblah Square on a Sunday afternoon to protest against the government's proposed new Carbon Tax. Under the new environmental tax, residents would be required to pay additional taxed based on vehicle mileage and blahblah.
Hope that helps.
Canberra, Australia - March 23, 2011: Thousands of protesters gather near blahblah Square on a Sunday afternoon to protest against the government's proposed new Carbon Tax. Under the new environmental tax, residents would be required to pay additional taxed based on vehicle mileage and blahblah.
Hope that helps.

