The study was set up to study healthy aging, and is managed by the Sax Institute in collaboration with the Cancer Council, the National Heart Foundation and NSW health agencies.
The Sax Institute is a coalition of 35 universities, public health and health service research groups.
Study co-author Professor Adrian Bauman, from the University of Sydney's school of public health, said the findings had important implications for public health programmes.
"They show that physical activity, while important, is no 'get out of jail free' card," he said.
Prof Bauman said programmes of the future might need to focus specifically on reducing prolonged periods of sitting as well as boosting the amount of exercise people undertook.
Earlier findings from the study had shown strong links between sedentary time and obesity.
But until now there had been limited evidence about the impact of sitting time on mortality risk.
Further work will be continued in a five-year follow-up study of 265,000 people enrolled in the research programme.