Australia says deaths are projected to outnumber births for the first time in the 2060s, as the country faces declining birth rates and an ageing population.

A government report forecasts population growth will slow to 0.9 per cent annually over the next four decades, compared with an average of 1.4 per cent over the past 40 years.

Treasurer Jim Chalmers said the number of Australians aged over 85 is expected to triple by 2066, increasing pressure on healthcare and aged-care services.