HOW MUCH CAN YOU SAVE?
At a discount of 40c a litre, a motorist buying 50 litres of fuel will save about $20.
"When you've got a big 40c docket, you might want to fill [the car] up totally so you're maximising the discount," Stockdale says.
The Foodstuffs fuel scheme, which covers participating New World and Pak'n Save stores, offers a standard fuel discount of 4c a litre if shoppers have spent at least $40.
However, drivers save very little under this rate - about $2 on a 50-litre fill - and would be better off using an AA SmartFuel card, which offers a standard discount of 6c a litre for $40 or more of petrol, Stockdale says.
Individual supermarkets may choose to offer a greater discount at different times. Last week, at least one North Island Pak'n Save was offering a 25c a litre discount if shoppers spent $200.
New World, Countdown and Pak'n Save supermarkets all offered a 50c a litre discount last year for customers who spent more than $400.
Foodstuffs spokeswoman Antoinette Shallue says fuel discount rates are set according to a variety of market conditions.
"[They] are a great way to support national, regional or store campaigns all designed to bring great rewards to our loyal customers."
A Countdown spokeswoman says fuel discounts are an additional way of rewarding shoppers. The promotions are highly valued by customers.
"We do see some customers changing their shopping behaviour to take advantage of these short-term promotions - particularly when the reward value is high (over 20c a litre)."
READ THE FINE PRINT
The Federation of Family Budgeting Services says people have to read the fine print around specials - on food and fuel discount offers.
"In some of the offers, to get the big amount off the petrol, you have to buy a certain amount of some brand product to qualify," chief executive Raewyn Fox says.
Shoppers benefited from a "price war" in recent weeks when the major supermarket chains offered the 40 cent discount if customers bought at least $150 worth of groceries. But the Countdown offer came with stringent conditions.
"You had to buy either nine Watties products in your trolley, or spend the $150."
And that could catch people out, Fox says.
The ensuing "price war" between supermarkets was great for consumers and demonstrated the importance of shopping around, Fox says.
HOW TO NAVIGATE IN-STORE SPECIALS
Basic rules such as sticking to a grocery list and leaving children at home will help avoid unnecessary splurging.
Looking out for expiry dates, especially on foods being offered at a cheaper price in large amounts, is also important, Fox advises.
Stockdale warns many petrol discounts expire after a period of 10 days - though others are valid for a month. "If you're not going to use them, then offer them to someone else."
HOW TO AVOID OVERSPENDING
Reward schemes such as Fly Buys combined with the promise of cheaper petrol can put some shoppers into overdrive.
Although many families can easily spend $150 at the supermarket, buying certain products to clock up reward scheme points or doing the groceries just to qualify for a special fuel discount offer, is unwise, Stockdale says.
"Think about pairing up with family members and saying, 'Let's combine our shopping - but I'll pay this week's bill and I'll get the voucher and next week it's your turn'."
It's about utilising the discount without overspending, he says.
