The vending machines in Melbourne seem to accept bills up to $20, sometimes $10, giving change. Older models are limited to coins, usually excluding $0.05 ones.
Does this change comprise of a mixture of bills and coins, or coins alone? One handicap in the States is that dollar coins are not widely circulated.
Edit: A datum, for example, is that I recall only one vending machine of the many (hundreds? possibly only dozens) I've encountered in the DC area accepting fives and offering change in dollar coins, and that (a) doesn't always and (b) is selling ice cream treats for US$1.75 each.
The other day I went to get some productivity-enhancement M&Ms from the candy machine at work. When I opened my wallet, I didn't immediately see a $1 bill. Then I looked some more and I found one, and I was happy! But of course that doesn't make any sense. If that bill hadn't been a $1, then it would have had to be a $5 or more, with an expected value of $5+, which is an amount that I certainly would not have paid for a bag of M&Ms, most excellent though they may be. This means that I preferred a bag of M&Ms to $1 (that's why I went to the candy machine in the first place), $1 to $5+ (I was happy when the bill turned out to be a $1), and $5+ to a bag of M&Ms (I wouldn't have bought them at that price). Not too surprising I guess, but still kind of weird.