Her top tips:
• Make a meal plan — A weekly meal plan means buying only what you need. "It's a stress-buster on so many levels. It helps rein in the grocery spend by reducing overstocking and eliminates the time wasted doing expensive emergency dashes to the supermarket.
"The plan can accommodate supermarket specials and seasonal variations and the dishes you choose can be selected to suit the age and stage of the household. The menu plan also makes it more easy and achievable to cook double batches, squeeze in some baking or get a head start on tomorrow's meal when today's is under control."
• Work out what's essential — For some, organic vegetables may be non-negotiable essentials while for others a bottle of wine is a rare luxury.
Gray says it is much easier to work out what is essential before you get to the supermarket. "It's very tempting. You see lots of things in a split second that are all very desirable. Being strategic about the way you shop and making decisions before you get to the supermarket are some of the most powerful ways to save money."
Start by listing the non-food essentials you need most weeks, such as toilet paper and the ingredients needed for the meals on your plan.
Then list the luxuries. Buy one or two items from your luxuries list every week and take care how you use them. Regular luxuries will keep you on track, she says.
"Otherwise it's like living on a diet, and you'll fail."
• Shop once a week — Supermarkets are designed to tempt you to spend more than you want to, so limit the number of times you go, Gray says. "If you run out of something, make do until the following week. You'll be amazed at what you can manage. You will use all the old cans of bean salad in the back of the pantry and learn to use the ingredients you bought for a special recipe you never got around to."
Gray says shopping online can help save money by eliminating the possibility of grabbing tempting treats, but you'll need to save more than the delivery cost.
• Track your spending — Record how much you spend each week at the supermarket so you can identify blow-outs and why they occurred. In a notebook or on your phone, list the products you usually buy and their prices. That way, when supermarkets offer a special, you'll see whether it represents a good deal.
• Eat with the seasons — Gray says it's cheaper and more interesting to eat seasonal vegetables. "Eating seasonally ensures constant variety. Instead of frozen peas every night, we might be eating corn and courgettes and aubergines and tomatoes for several months, then more broccoli and cauliflower, celery, pumpkin and sprouts."
• Shop around — She says supermarkets are rarely the best places to buy produce or meat. Local suppliers are usually cheaper, even after factoring in the cost of petrol and time in getting around. "Every single study we've done shows you can save thousands of dollars a year by buying at the butcher or the fruit and vegetable shop."
• Pay cash — Before you reach the supermarket, withdraw cash. Gray says people who shop with cash spend an average of 20 per cent less than those who use a card. Save any change to pay off debt or buy things in bulk when you see them on special.
• Shop smarter — Gray says people shouldn't buy anything they could make. "Ready-made muffins are a luxury but you could make them at home and bung them in the freezer."
Avoid convenience foods and heat-and-eat sauces — buying the ingredients will usually end up being cheaper. Make and freeze things like lasagne rather than buying them in ready-to-heat containers.
But determine what works for your family — some convenience foods, like stock, tomato paste and frozen vegetables are cheap enough so it makes sense to buy them.
She recommends looking for supermarket home brands. "We use the basic range for staples like flour and milk and the supermarket's named range for most other items."
Meat is a big part of many families' supermarket bills — Gray recommends reviewing the cuts to see if they are budget-appropriate. A butcher can help you learn how to use cheaper cuts of meat.
But she recommends quality over quantity in many cases. "A few 'made from actual meat' sausages will make a lovely cassoulet, risotto, sausage-and-onion tart, hot pot, or pasta."