Is it possible for an Australian student to study journalism at Columbia University in New York City? N?

It's definitely possible, though very expensive if you're accepted. Plan on a minimum of 45,000$ per year, not including airfare, trips home, and other little extras. Tuition and fees are around 42,000$ alone, and then you have to think about your living situation (NYC is extremely expensive, and has a very high standard of living). You'll need a student visa, along with all other documentation required to receive that. This will likely include a birth certificate, passport, all the usual things, along with proof that you can afford to sustain yourself financially. This means that you or your parents really do need money in the bank, and lots of it. You'll need to take the SAT and/or ACT, which is a test required of all students in the US to apply for college. Obviously you'll need to complete an application, essay, etc. and submit it in due time (long before the deadline).