The number of New Zealanders needing knee replacements is expected to skyrocket in the next 20 years - and researchers say much of this is to do with our alarming obesity rates.

One orthopaedic surgeon told the Herald it would not be possible to perform the required number of surgeries each year, and the demand would likely lead to long waiting lists - with patients waiting as their condition worsened.

The associated healthcare costs - including replacement surgery, GP visits, pharmaceutical drugs and physiotherapy - were estimated to rise from $199 million a year to $370m by 2038.

A University of Otago study, published in the New Zealand Medical Journal today, used data from the 2013 Census to make predictions about the growing demand for the surgery due to knee osteoarthritis.

In 2013, 5070 knee replacement surgeries were undertaken in New Zealand. As the population aged and obesity rates increased, researchers said this number was expected to climb to 9040 by 2038.

Each publicly-funded surgery cost about $20,000.

Led-author Dr Ross Wilson, from the university's Centre for Musculoskeletal Outcomes Research, said these findings were a huge concern because it showed the growing burden on the health system to meet this increasing demand.

"We know already with the prevalence of osteoarthritis there is a great strain on our ability to provide joint replacement surgeries.

"With this projected growth, we are going to need to invest a lot more into being able to meet those health needs," Wilson said.

The majority of people needing knee replacements were overweight or obese, he said, because often the disease was an inflammatory response to excess fat on the joints.

"Therefore, there is a number of public health measures we can do to help slow the rising burden," Wilson said.

It reinforces the importance of encouraging New Zealanders to make healthy choices about what they are eating and to increase their levels of exercise, Wilson said.

More resource into early interventions, such as exercise therapy to alleviate symptoms, was needed and could help reduce long-term costs, he said.

David Gwynne-Jones, an orthopaedic surgeon at Dunedin Hospital, said it was unlikely 9000 surgeries would even be possible over the space of a year, which would mean more people would miss out.

"This is a projection of the demand but it is probably going to get to the stage where rationing gets harder and the disease will have to get a lot more worse before those in need can get a publicly-funded surgery."

On average, patients needing knee replacements had a Body Mass Index (BMI) of 32, which was classed as obese, Gwynne-Jones said.

"It is difficult to say obesity causes osteoarthritis, but there is definitely a strong link and these findings do not surprise me, they just reinforce the international research.

"We used to do two hip replacements for every knee, but now we do far more knees and we know this is also the case in the US," Gwynne-Jones said.