She used an app which kept count of how many calories she consumed.
While Ms Hay, of Remuera, didn't weigh herself to see if there were any changes, she did feel more confident.
But unlike Ms Hay, 81 per cent didn't bother changing their habits to get a summer-ready body.
Ali Bensemann was among those who didn't change their lifestyle in the lead-up to summer.
The 19-year-old, of Ponsonby, already eats healthily and goes to the gym about three times a week.
"I enjoy food too much to diet," Ms Bensemann said.
Plus, both her parents are doctors. "So I know better than to crash diet."
The Heart Foundation's national nutrition adviser, Delvina Gorton, said crash diets were created by people wanting to "get rich, quick".
Ms Gorton said it was easy at this time of year for an extra kilo or two to sneak up.
Weight loss often featured in people's New Year's resolutions, and crash diets could seem like a "really easy" solution.
"They often sound quite convincing and appealing."
Ms Gorton said anyone trying to lose weight was at risk of getting sucked in.
One of the diets du jour is the Dukan Diet, reportedly followed by the Duchess of Cambridge.
The high-protein diet involves four phases - including one which allows only one serving of fruit, two slices of wholewheat bread, some starchy food and 1.5 ounces of cheese daily.
Ms Gorton said highly restrictive diets were dangerous because they tended to lead to yo-yo dieting in which people became trapped in a cycle of losing and gaining weight.
As well, as soon as a person finished the diet, they regained the lost weight and sometimes put on more.
Instead, Ms Gorton recommended the "totally unappealing" way of sensible and healthy eating with about 60 minutes of exercise a day.
"It's really just about eating a bit less of things other than vegetables and fruit, choosing healthier foods and getting out and moving more," she said.
Loose weight the healthy way
* The best way to lose weight is reducing your kilojoule intake by cutting down on fat and sugar.
* Do regular exercise.
* Try not to lose weight too quickly. Aim to lose it slowly and steadily.
* Between half a kilo and two kilos a month is healthy, and you are more likely to keep the weight off for good.
* Crash diets are not a healthy option. Losing weight quickly involves losing essential water and muscle as well as fat.
* When you stop the crash diet you are likely to not only regain the weight you have lost but you may also put on even more weight.
* You are more likely to succeed with the support of family and friends.
Tips from the Heart Foundation
