Consumer New Zealand is warning against buying with a retailer simply because you have a loyalty card, as competition intensifies.
Countdown's Onecard has joined with AA Smartfuel to become the country's largest loyalty scheme.
Consumer New Zealand chief executive Sue Chetwin says it could be that people are becoming more disloyal.
"They've got wallets full of loyalty cards, so that they're not attracting people into the stores or into the band. People are just using them once they buy stuff.
"If they are doing that, then that's probably the most sensible way to be using them."
She says it takes quite a long time to build up rewards, or whatever it is a scheme is promoting.
"You are far better to just buy stuff on price, and then use the loyalty scheme as a nice bonus if you've managed to get something at a reasonable price."
The new Onecard scheme is soaring above Fly Buys, with its combined 3.5 million card holders.