I did look into gift cards a few years ago, but it proved to be unworkable at that time, and I doubt anything would have improved in that regard. From what I have read, you used to be able to use such a method and then they cracked down on a few loopholes.

There is also a measure of trust involved, and no way to test any claims, and not being from the USA I don't know anything about the institutions.

And of course, with small items like ebooks you have to factor in any fee, and my recollection is they made it not worth it. If money is no object and you just want access, then that might be okay, but that's not who I am ... I only believe in paying fair prices, no exceptions.

All that said, if you know of a trustworthy American credit card company that sells gift cards to Aussies, then I am happy to look into it again.

P.S. Unlike with normal Amazon purchases, Kindle ebooks are a one-click instant purchase, no cart involved, and even to grab free ebooks from Amazon, they require a credit card be linked to your account, so I am not exactly sure of how I would get around that with a non Amazon Gift Card.
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I really don't know how a card purchased in the States and registered to a random American address would work under a profile with foreign information tied to it, but I would imagine that if you were able to get someone to purchase a $100 gift card, I use American Express, and send you the numbers you could attempt to use it on any storefront, I've never had it rejected at a digital store. I'm not sure if this is really any easier than asking someone to buy you a gift code for a game you would want but depends on the person and the need I suppose. As far as Amazon, I believe one click purchases automatically use your default payment method so you would have to assign the gift card as the default before clicking to buy. Never even using a foreign storefront, however, I really can't say if any of this would work, just adding what little knowledge I have in regards to using gift cards as a purchase method at a number of storefronts.