Credit ratings agency Standard and Poor's looked at the possible impact of an aging population on New Zealand's finances. 

On current projections, the population would grow from four million last year to 4.8 million by 2050. The working age population would fall, however, from about 66 per cent to 60 per cent. 

Standard and Poor's credit analyst Kyran Curry said the aging population would put pressure on public finances as demand grew for health and long-term care services and social security pensions. 

That would be only slightly offset by lower outlay for unemployment benefits as the decline in the workforce reduced the number of jobless. 

New Zealand's old-age dependency ratio – the number of retirement-aged people as a proportion of the working population – would rise slower than the global average, but would still reach 39 per cent by mid-century, up from 19 per cent last year. 

Globally, the old-age dependency ratio is estimated to surge from 19 per cent last year to 45 per cent in 2050. 

Though governments adapt to change, Mr Curry said if New Zealand social policy remained the same the amount spent on age-related matters such as healthcare and pensions would rise from 12 per cent of gross domestic product in 2005 to 21 per cent in 2050. 

Because the government would have to borrow more to pay for the costs of an aging population, total government spending would rise to almost 46 per cent of gdp from 33 per cent now. 

But Mr Curry said because the New Zealand Government had not been spendthrift and had a strong balance sheet, it had more flexibility to respond to the demands of an aging population. That could include early investment in healthcare and pensions provisions. 

Many European countries badly in debt would not be in such a good position to cope, he said. 

The world's population is expected to hit nine billion by 2050, from six billion in 2000. 

The United Nations estimates that by the same date, the median age will increase by 10 years to 38.