"Inactivity, unfortunately, has terrible health impacts," Prof Machado, of the University of Otago psychology department, warned.
"It’s something we really need to address."
An international obesity update released last year showed adult New Zealanders are the third-most obese in the OECD, behind the United States and Mexico, she said in an interview.
And New Zealand national surveys show that physical inactivity among New Zealanders aged 60 and over is rising — from 18% in 2012 to 22% in 2014.
Prof Machado, who directs the University of Otago neuro-science programme, said that inactivity adversely affected brain health, reduced thinking ability and increased the risk of neurological diseases.
"Inactive" adults were undertaking less than half an hour of physical activity, such as walking, each week.
Prof Machado told a recent community seminar on "Ageing Well" that people could significantly boost their brain health through some regular brisk walking or other exercise. The free seminar was organised by the Otago-Southland branch of the New Zealand Psychological Society, as part of National Psychology Week.
Prof Machado, of the Otago psychology department, said a recent study of 32 Otago University students, conducted through her laboratory, showed that 10 minutes of exercise, including "vigorous stair climbing", improved their performance in the most difficult of cognitive tasks.
And volunteering could also enable older adults to "optimise their brain function", including through the stimulating social contact.
A recent Otago study of 91 retired adults, aged between 65 and 75, showed community volunteers were engaged in more cognitive and social activity, and performed better in reasoning tests, she said.