The results showed increased secondhand smoke levels at indoor tables next to the smokers' area during times of "high smoking patronage", like weekends and in summer when most restaurants had all the doors and windows open.
Secondhand smoke levels in areas furthest from doors and windows were much lower.
Nick Wilson, an associate professor at Otago University's Wellington-based department of public health, said secondhand smoke exposure was measured according to the number of "fine particles" of smoke found in the air at restaurants.
These were "the most hazardous smoke particles and most likely to cause cancer and heart disease," he said. "They could not be coughed up like larger smoke particles and went deep into the lung."
Other findings highlighted the importance of restaurant design, with higher levels of indoor secondhand smoke found at eateries with enclosed smoking areas.
Action on Smoking and Health director Stephanie Erick said the research showed a total smoking ban at restaurants and pubs would benefit workers and patrons. "This is a health and safety issue."
But Hospitality New Zealand president Adam Cunningham said restaurant and bar owners needed clear guidelines about how to make outdoor smoking areas safe.
"The best outcome for society is not to ban smoking in outdoor areas. It's to help people who have outdoor areas to understand what's required of them to keep everybody safe."
Associate Health Minister Tariana Turia said the research proved a smoking ban at restaurants and bars was needed.
"I'm not surprised at the findings, having experienced smokefree areas and knowing that smoking in close proximity, still the smoke gets in.
"This is about protecting workers as well as patrons," she said.
Prohibiting smoking at restaurants would benefit everyone and help make NZ smokefree by 2025.
"Tobacco is a significant health hazard," Mrs Turia said. "It costs the taxpayer an exorbitant amount of resource. I accept that smoking is an addiction [and] I feel for those who are smoking and understand the difficulties in giving up."
Representatives from Philip Morris, British American Tobacco and Imperial Tobacco said they would review the findings, and advocated a balanced approach to smoking restrictions that would enable smokers and non-smokers to enjoy public areas.
The study, led by Frederieke van der Deen from Otago University's Wellington campus, involved researchers from New Zealand and Britain.
