The Lincoln researchers' analysis of Health Ministry survey data shows that from 2003 to 2007 the average amount of time spent by men doing moderate or vigorous physical activity reduced by 39 minutes, and 25 minutes for women.
Yet brisk walking rose by just over an hour on average for men and 37 minutes for women.
Men tend to do a lot more physical activity than women.
One of the downsides of the reduction in strenuous activities is, unsurprisingly, an unwelcome contribution to what many experts call our obesity epidemic. In the decade to 2007, the average New Zealand man grew by 4.7kg and the average woman by 3.6kg.
Lincoln's associate professor of exercise and sports science, Dr Mike Hamlin, said the changes in physical activity patterns were important.
The reduction in moderate or vigorous activity might seem small at only around 5 per cent a week, but it all added up - to around 26 fewer hours each year.
Professor Hamlin calculates that each minute of moderate or vigorous activity expends roughly 7.5 kilocalories of energy.
"That's 11,700 kilocalories per year that is not expended ... There is about 7000 kilocalories in each kilogram of fat stored in the body, so, working this through, that's an increase over a year of 1.7kg of fat which could be solely due to a 30-minute drop in moderate/vigorous-intensity physical activity per week - presuming all the excess energy you consume went to fat.
"So you can see how, over a long period of time, a relatively small drop in activity levels can have a major influence on body weight, body size and [population] obesity levels."
Professor Hamlin said the ministry's 1997 survey did not include data on physical-activity levels, but it was likely, given New Zealand's long-term increase in obesity, that the decline in moderate or vigorous activity was also a long-term trend.
The rate of obesity has more than doubled since 1977, a change reflected throughout most of the developed world. On the latest survey data, from 2008-9, 28 per cent of New Zealand adults were obese and 37 per cent were overweight - only 34 per cent were normal weight.
Obesity is a risk factor for heart disease, type 2 diabetes and cancer.
There is debate among researchers over the relative contributions to population obesity of changes in diet and changes in patterns of physical activity and sedentary behaviour such as watching television and using computers. Many public health specialists argue that the west has become an "obesogenic environment" where it is natural that obesity will increase until we address the marketing and manufacturing of unhealthy food, our dependence on motor vehicles and the supersizing of deep-fried takeaways and sugar drinks.
The Lincoln analysis will be presented at the Australia and New Zealand Obesity Society conference in Auckland this week when delegates will hear about the latest research findings on obesity - from how preterm-born baby boys grow into men who are fatter than others, to the latest ways to encourage children to eat fewer calories and more fruit and vegetables.
Professor Hamlin said any physical activity was helpful, but it was important to do some that increased the heart rate.
"You've got to get your heart rate up a little bit more, even if you walk a little more briskly or go swimming or jump on a bike."
He said the relatively small size of backyards at modern homes contributed to the problem of obesity.
"There's no room for a garden, no room for the kids to play, no room to do any outside activity. Those sorts of things do play a role, that incidental activity ... in the garden, walking down to the shops rather than going in the car. All those things will cause the changes that we are seeing."
