Some district health boards are not doing enough weight loss surgery for morbidly obese patients, a researcher says.

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Bariatric surgery, which involves stomach stapling and a gastric bypass, can not only lead to lasting weight loss but also put diabetes into remission, and reduce the risk of cancer and heart disease.

But research shows the DHBs differ widely on how many bariatric operations are done.

The Canterbury and Waitemata boards have similar populations and obesity rates - but Waitematā does 104 of the operations each year while Canterbury does only 13.

Rinki Murphy, who is an associate professor at Auckland University, said the study suggested access to such surgery was equivalent to a "postcode lottery".

"So it's simply showing that that variability, according to the DHB regions, needs to be looked at and we need fairer processes to make sure that everyone has an even chance of accessing this very important treatment."

As many as 50,000 people could benefit from bariatric surgery, she said.