THE number of Australians with dementia will quadruple by 2050, rising from 245,000 today to 1.1 million, with dire consequences for the health and aged-care systems, a report says.

More than 75,000 baby boomers, the first of whom turn 65 next year, will have dementia by 2020 unless there are significant medical breakthroughs.

The report, by Access Economics, was commissioned by Alzheimer's Australia, and uses new census data and other evidence to revise up by 55 per cent dementia projections made four years ago.

It points to huge shortages of people in coming decades - in families and the workforce - able to care for the predicted numbers of people with dementia.

''This report makes clear that the dementia epidemic is dramatically worse than we thought. It's a devastating wake-up call to everyone involved in health-care planning,'' the chief executive of Alzheimer's Australia NSW, John Watkins, said.

Every week more than 1300 cases were diagnosed, the report said. By 2030 this was predicted to rise to 3600, and by 2050 about 7400 new cases a week were expected. Many more people were likely to have some form of cognitive impairment that did not reach the dementia threshold.

Dementia is already the single leading cause of disability in Australians aged 65 and over. The ageing of the population is the main driver, with the number of Australians over 60 rising from 4 million to 10 million by 2050, the report shows. Also, the rate of dementia in the population is higher than was previously thought.

The chief executive officer of Alzheimer's Australia, Glenn Rees, said the study was not alarmist but was based on ''middle of the road'' assumptions.

The report's author, Lynne Pezzullo, said that ''within our lifetime'' dementia and other neurodegenerative diseases would overtake disorders such as cancer and cardiovascular disease as the major threats to health and quality of life.

But the report shows that the picture need not be quite so grim if more people exercised, stopped smoking, and controlled their blood pressure.