Over-indulgence can add an extra half a kilogram to the scales, and in many cases up to three, and the weight gain is not likely to be reversed.
The findings come from US specialists, but local dieticians warn that obesity trends at home show Australians have the same susceptibility.
"Those extra kilos are so much harder to get off than they are to put on, so moderation is the key over Christmas," said Sonya Stanley from the Dietitians Association of Australia.
"Even an extra 600 kilojoules a day - the amount in a small slice of Christmas cake, a full-strength can of beer or a marshmallow Santa - can result in a couple of extra kilos come January."
She said festive eating habits made it unrealistic to try to lose weight in the holiday season, with weight maintenance the best goal.
The rapid weight gain was being driven by a triple whammy of larger food portions, more alcohol consumption than normal and more snacking on sweet and fatty foods.
But Christmas need not bring a Santa-like stomach, Stanley said.
"It's easier to overindulge during the festive season, but if you can limit the days you overeat to Christmas day and one or two parties, you'll have better control over your waistline," she said.
The key was to be selective and choose the healthier option most of the time, like fruit and yoghurt in place of pudding, or mini-kebabs over party pies, and sushi over fried spring rolls.
"And instead of a glass of wine, try a wine spritzer," Stanley said.
A recent study showed Australia was now the official heavyweight champion of the world, with a greater proportion of obese citizens than the United States.
Middle-aged Australians are leading the way, with seven in 10 men and six in 10 women aged 45 to 64 now registering a body mass index (BMI) of 25 or more.
