Until September 30, Coles is offering 20% off mobile phone recharges if you spend more than $100 on groceries. That's a good potential saving, but will probably lead to grumbling that Coles is exploiting its market power.
The ACCC has said in the past that these kinds of deals are permissible, provided they're not offered as an occasional loss leader and not a permanent discount. As ever, spending $100 just to save money would be pointless, but that's not a particularly large shop in the first place. Just don't be tempted to stock up on a huge recharge if your plan will eliminate unused credit after 30 days.