A report by the Organisation for Economic Co-operation and Development says poverty in New Zealand almost doubled between the mid-1980s and 2005.
Poverty is defined as having to live on less than half the median income.
The report says it grew from 6% of the population to more than 10% in that time.
The BBC reports the cumulative rise in inequality across the 24 OECD countries where data was available, shows the cumulative rise in inequality was 7% over the past 20 years.
But the OECD said this was not as large a rise as had been expected.
It also found income inequality in New Zealand to be high.
The National Party is attributing poverty to having large numbers of people on benefits, and says boosting employment is the best way to combat this.
However, the Labour Party says National's policies caused the problem in the first place by catering for a wealthy few.
The report does not cover developments in the past three years.
