Lead author Dave Monro, the Heart Foundation's food industry setting manager, said it was positive to see reductions in low-cost staple family foods.
"Bread, for example, is the leading source of salt in the New Zealand diet, accounting for around a quarter of our salt intake. It's encouraging to see the sodium content come down by 14 per cent in this product."
But salt in butter increased by 21 per cent, at the same time as the sodium in margarine was cut by 19 per cent.
Salt levels rose by 2 per cent in cheese products, and 6 per cent in corned beef products.
"Food companies need to make more effort in these areas," Monro said.
About three quarters of dietary salt comes from processed foods, so cutting salt in the food supply is crucial to reducing intake, the study authors said.
Co-author Dr Helen Eyles from the University's National Institute for Health Innovation said there needed to be greater focus on food groups that had not reduced sodium since 2003.
"This is essential if New Zealand is to meet its commitment to a 30 per cent relative reduction in population sodium intake by 2025."
Dr Eyles said New Zealand needed a government-led salt reduction plan, as had occurred in the United Kingdom and Australia.
"Such a strategy should include targets for manufacturers, but also improved labelling and a public awareness campaign highlighting the impact of salt on health," she said.
The research paper, Changes in the sodium content of New Zealand processed foods: 2003-2013 was published in scientific journal Nutrients.
