The cost of Australia's ageing population will eclipse Medicare three years earlier than expected, as baby-boomers retire and the cost of the aged pension, aged care and health balloons to more than $36 billion a year.

In a warning the day before Treasurer Josh Frydenberg officially cuts Australia's migration cap by 30,000 a year in Tuesday's budget, one of the government's top independent advisors found migrants have softened the costs of the ageing population and had a "profound influence on Australia’s population structure".

Treasurer Josh Frydenberg will deliver his first budget on Tuesday. Credit:Dominic Lorrimer

"Changes during the next couple of decades will be particularly significant," the Parliamentary Budget Office found, as Australians born between 1946 and 1964 move from retirement, to aged care and then die out.

"The ageing of the baby boomer generation will affect the budget in waves."