Dick Smith has 20 per cent off iTunes cards until Monday (16 September). Delivery from the web site is free.
That's a better deal than the one currently running at Target. It doesn't beat the 25 per cent off you might get at EB Games, which has a two-$20-cards-for-$30 deal, but that depends on whether your local branch has stock. That said, stock of the cheaper cards from Dick Smith already appears to be low as I write this, so get in fast if you're tempted.