Not sure if they're always done it, but recently noted the local Paknsave petrol was 5c more expensive than a petrol station around the corner. Had never really looked at their prices before. Then it occured to me that the requirement of spending $70 in store for a 6c discount voucher actually only gets you a whole 1c off a litre at the pumps. What a deal.
Pak 'n Save petrol in Wellington is quite often cheaper than Z/BP/Mobil/Caltex and you get 6c off with no minimum spend.
I now go there most of the time since I refuse to give a cent to Z unless it's absolutely necessary. IMHO they're now more evil than Google and Uber combined..