The overweight and obese are at much greater risk of getting a severe bout of the flu that will land them in hospital, a large Australian study has found.

Researchers at the University of New South Wales say the obese appear to be just as vulnerable to the seasonal flu as those with cardiovascular disease and diabetes and must get their flu shot.

Using the Sax Institute's 45 and Up Study, the researchers examined the health records of nearly 250,000 NSW adults from 2006 to 2015 to investigate if there was an association between body mass index - a general measure of obesity - and flu diagnoses.

"We essentially found that people who were considered either overweight or obese had a higher risk of having a laboratory confirmed flu diagnosis," said lead researcher Associate Professor Better Liu, a medical epidemiologist at UNSW.

The more overweight the person was the greater their risk of flu and flu-related hospitalisation. For every five-unit BMI increase above 22.5 there was a 15 per cent increase in risk of having a diagnosis of influenza and 42 per cent increase in hospitalisation, the study found.

"The very obese people (those with a BMI between 40 to 50) had the highest risk. The risk in the very obese was about 70 per cent higher than people of a healthy weight," said Professor Liu.

The findings are published in the International Journal of Obesity.

It wasn't until the swine flu pandemic in 2009 that people started recognising that the overweight were at higher risk of getting that particular strain of influenza.

It's thought that carrying excess weight impairs the immune system's response to influenza infection, write the authors.

This study shows the overweight and obese are at higher risk of all seasonal flu strains, explained Professor Liu.

"Really the message is that if you are overweight and certainly if you are very obese you really should go and get vaccinated to protect yourself," said Professor Liu.