The Salvation Army says basic food costs have increased faster than the rise in food prices as measured in official figures.
Every three months, the Salvation Army visits the same supermarket in South Auckland to calculate the cost of a basket of food a typical low-income household might buy.
This includes basics such as fresh fruit, dried food and cleaning products.
The survey found the cost of living for a single-parent family has risen 9.1% in the past year and 8% for a two-parent family.
In the latest Food Price Index, the cost of food in the year to the end of June was 7.5%.
Salvation Army social policy spokesman Campbell Roberts says the results for South Auckland would easily translate to other parts of the country.
He says in worst-case scenarios people have to make choices how many times they eat in a week.
Campbell Roberts says New Zealand is failing to address its growing poverty problem and the Government must provide more financial support to struggling families.
Mangere Budgeting and Family Support Services chief executive Darryl Evans says the service has seen families so desperate to eke out food for the week that they padlock their fridges to stop their children eating when hungry.
Mr Evans says many working families do not qualify for Government support and cannot approach Work and Income for food grants if they have spent all their wages on rent.