They're back! They've been unavailable for around a month now, but G2A once again has cheap $100AUD Xbox Live cards.
Go to the G2A link, change to AUD, and you should see a "Digital Key" for $72.33. Don't buy this - buy the one below, where it says "1 Offers from $65.75". Add that to your cart, untick the "G2A Shield" option, and then choose paypal to pay (which adds ~ $3 to the price).
I've just bought 3 with no problems - YMMV. I have no affiliation with the dealer (and in fact a general level of mild angst buying from sites such as G2A) so although my purchases worked out OK, there is a non-zero level of risk when buying from G2A. However, 3 successful transactions in a row suggests low risk.
Enjoy!
Just a heads up, can't use xbox credit to buy xbox gold.
Be wary about using G2A, they attempted to use my paypal to purchase things from hong kong without my consent.