Dementia is on track to become Australia's biggest killer, but new research shows rates are actually declining in older Australians.

Key points: The study looked at two groups of older Australians in home and long-term care

The study looked at two groups of older Australians in home and long-term care It found the rate of dementia decreased in both groups

It found the rate of dementia decreased in both groups An expert says it may be the first example of a decline anywhere in the world

A large-scale research project looking at Australians accessing home or long-term care services, published in The Journal of Gerontology Medical Sciences, has found the country's dementia rates have fallen.

Researchers now believe there could be a need to reassess current estimates of the disease.

The country's ageing population means the overall number of older Australians with dementia, and people accessing aged care, is still set to increase.

However, researchers believe public health measures, changing lifestyles, higher education rates and a decline in smoking are all contributing factors to the drop in the rate of dementia.

South Australian Health and Medical Research Institute (SAHMRI) Doctor and lead author of the study Stephanie Harrison said the findings were consistent with other studies in high-income countries including the Unites States and the United Kingdom.

Data was evaluated from the SAHMRI-based Registry of Older South Australians (ROSA) as well as other sources, including information from people who have accessed aged care services nationally.

The research project has looked into access to home or long-term care for older Australians. ( AAP: Glenn Hunt )

The study of 188,846 older people receiving home care services found the prevalence of dementia fell from 26 per cent in 2005 to 21 per cent in 2014.

For 348,311 older people starting long-term care, the dementia rate fell from 50 per cent in 2008 to 47 per cent in 2014.

The research did not show exactly why the rate of dementia had fallen, but could indicate the prevalence of dementia might be changing.

"The great thing about the ROSA project is we are using data that is already being routinely collected about older people, so when people want to access aged care, they have to have quite a comprehensive aged care eligibility assessment which collects a lot of information about the individual," Dr Harrison said.

"What we're doing at ROSA is linking that information with other information we have about health, including what medications people are using, what services they are using and information about mortality to create this rich new data source."

Lifestyle change can reduce cases of dementia

Associate Professor Michael Woodward, who is Dementia Australia's honorary medical advisor and specialises in geriatric and rehabilitation medicine, said it had already been demonstrated that public health measures and changes in individual lifestyle practices could reduce the number of new cases of dementia.

An expert said the findings may be the first example of a decline in dementia rates anywhere in the world. ( ABC Central West: Melanie Pearce )

"I'm not aware of any other study that's shown a reduction in the prevalence of dementia which is the absolute number of people in a setting, I find that hard actually to fully believe because reduced incidence is only going to translate to reduced prevalence over many years," he said.

"But the fact that it has been shown, it may be the first example of this anywhere in the world, it's very, very exciting.

"So I am, and I think the whole dementia community, in fact the whole Australian community should be very, very excited by the fact that we are showing a reduced number of people with dementia in the settings that were examined."

He said it would take a long time for the numbers to reduce, based on changes in individual lifestyle and public measure.

He said as the population changed there would be a higher demand for care even if dementia numbers began to decline.

"People are hoping at some stage we'll develop effective treatments for dementia," he said.

"That's still a way off, there's some very promising drugs in development but our emphasis on this stage should be on prevention."

The first set of hearings during the Aged Care Royal Commission in Adelaide last month revealed dementia was set to become the biggest cause of death for people aged over 85.

The commission heard that the condition — which does not have a cure and cannot be treated — would impact every Australian and create new challenges in the community.

Dementia Australia chief executive Maree McCabe told the commission there were about 436,000 Australians living with dementia today.

She said by 2050, that number would surge to 1.1 million.

'It's horrible to watch, it's heartbreaking'

Madi Salter spent eight weeks caring for her grandmother, Margaret, who has Alzheimer's disease after her grandfather, Bill, was admitted to hospital on Christmas Eve after a fall which resulted in several brain bleeds, a fractured skull and then a stroke.

Bill had been Margaret's full-time carer.

Madi Salter and her family will take part in Dementia Australia's Memory Walk. ( ABC News: Helen Frost )

"We had no idea just what Pop was doing every day, all day, looking after Gran and supporting her and just making sure she was safe," Ms Salter said.

"It's horrible to watch, it's heartbreaking I guess, to see someone you love slowly get to that point where they don't remember things.

"She didn't remember what had happened to Pop."

Since her grandmother's diagnosis two years ago, Ms Salter and her family have changed their lifestyle and become a lot more conscious of their health.

"My [other] Nanna also has Alzheimer's, so we're very aware of it in our family," Ms Salter said.

"[We are] just trying to eat well but also a couple of nights a week go for a walk together and also Sudoku and things like that.

"I enjoy those kind of mind games and teasers and stuff so trying to keep my mind working and things like that are important."

Ms Salter and her family will take part in Dementia Australia's Memory Walk in Glenelg, west of Adelaide, this Sunday.

Initially, her goal was to get the family walking together and to meet other people who were going through the same thing.

"We've raised almost $2,500 and it's been incredible the support that we've been given through family and friends and work and the community as well," she said.