An Australian study involving more than 450,000 smokers has found that the weight gained by those who gave up smoking was significantly less than expected.

The study conducted by Tasmania's Menzies Institute for Medical Research surveyed 388,432 daily smokers and 63,403 people who'd recently quit smoking and compared their weight differences over a five-year period.

Results showed that, on average, those who quit smoking gained an average of 4.1 kilograms over the life of the study, while those who continued smoking gained just 1.5 kg, meaning the overall weight gain for the "quitters" was a surprisingly small 2.6 kg.

In the past, many people have cited the possibility of major weight gain as a reason for not kicking the habit.

In a news conference on Thursday, the report's authors, Jing Tian and Seana Gall, stressed they did not want the findings to be taken the wrong way.

"We don't want our findings interpreted as an incentive to keep smoking," said Gall, a cardiovascular epidemiologist, in their statement.

"Other studies suggest that this small amount of weight gain does not offset the many health benefits of quitting smoking."

The authors could not identify why people who quit put on more weight than those who did not.

"The reasons for the weight gain after smoking are complex, but probably related to changes in brain activity and metabolism after quitting," Gall said.

Abby Smith, director of Quit Tasmania, an anti-smoking group, echoed the authors' views saying, "Smoking causes cancer. The best approach to tackling weight gain is by being active and eating a healthy diet."

Cigarette smoking is responsible for about 6 million deaths worldwide each year, and is believed to contribute to most Australian smokers dying prematurely.