The Opposition is accusing the Federal Government of preparing to break a key election promise and cut the age pension.

It follows Treasurer Joe Hockey's warning that welfare must be a "safety net" and not a "cargo net".

Mr Hockey sounded the alarm in a speech called the Global Age of Responsibility which he gave in Washington this week.

"Many developed economies are ... facing a set of longer-term structural shifts arising from the ageing of our populations," he said.

"Australia is no exception. We are an ageing society and our birth rate does not replace our population."

The Treasurer said that has led to a "demographic bulge" confronting the economy and the budget, "which governments have chosen to ignore for too long".

Mr Hockey said the ageing population would "inevitably" have an impact on the affordability of "health care, aged care, pensions and discounted services of older communities".

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He said Government spending on the age pension would increase by about 70 per cent over the next 10 years.

"We need to ensure that access to the pension system is prioritised for those most vulnerable," he said, noting the International Monetary Fund (IMF) argument to raise the pension age is a "worthy consideration".

Mr Hockey said neither increasing the birth rate nor migration were immediate solutions.

He said the Government's priority was driving economic growth in the long-term, but difficult decisions would have to be taken in the short term for that to happen.

"Achieving long-run fiscal sustainability will require winding back some spending that our population have come to take for granted," Mr Hockey said, without specifying what type of spending would be cut.

Opposition spokeswoman for families Jenny Macklin told AM the Government was preparing to cut welfare payments it promised before the election not to touch.

"So if it happens that there are cuts to pensions or changes to pensions in the upcoming budget, this will be a complete betrayal by Tony Abbott to 2.3 million Australian age pensioners – a total betrayal," she said.

"These are people who are elderly, who are living on around $20,000 a year."

Ms Macklin said the Government's paid parental leave scheme, which will cost the budget $5.5 billion per year, should be the first welfare scheme to be put on the chopping block.

"The warped priorities of Tony Abbott will see wealthy women in his budget get $75,000 to have a baby," she said.

"That's where the cuts should come."