Someone commented earlier that the Woolies gift cards are useless for this purpose, but I've just thought of a reason why they might actually be useful.
I'm in a situation where I might be short of the KF miles required for a reward booking. One option is to pay to top up my KF miles at a cost of $40 per 1,000 miles. Or, I could effectively pre-pay my grocery bills for 6 months by purchasing gift cards from Woolies. E.g. buy $15k worth of gift cards and get 45,000 KF miles. Assuming I spend $600 per week, which otherwise would sit in my offset account until required, I've worked out the interest cost of doing this to be less than $200. That compares pretty favorably to purchasing 45,000 miles for $1,800 from the airline!
Even if you spend a lot less than $600 per week on groceries, it still could work. E.g. with a spend of $300 per week it would take you almost a year to use all the gift cards, but the interest cost would still be less than $400.