Processing a refund for a fare of a few hundred dollars MUST be more expensive than paying today's microscopic interest rates. The only possible win for an airline would be breakage, i.e. people failing to claim their refunds.
We're not talking about a few hundred dollars, flights from Australia to Europe or the US are closer to $2K (cheapest economy, and north from there). No doubt they will expect some breakage, and more importantly they'll expect (and if necessary incentivise) people to retain credit rather than claim a refund.
Maybe I'm being too cynical, and Alan Joyce knows something we all don't, so is happy to keep selling these flights. Or I'm not being too cynical and it is boosting cashflow into the airline.
EDIT: Since I posted this a short time ago, they do indeed to have stopped selling flights in Nov/Dec. I am sure I looked two hours ago and saw QF1/2 on sale, now it is not.