Dewes grew up in a "conservative family" as one of eight siblings. After leaving school, she studied music at the
University of Canterbury and became a music teacher at
Epsom Girls' Grammar. Part of the school music curriculum was a song of lament about
Hiroshima and
Nagasaki atomic bombings. She joined a non-violent waterborne protest group called the Peace Squadron, aimed at preventing armed US warships from visiting
Auckland Harbour. During the late 1970s, she and a growing number of New Zealanders rallied against the
United States Government's policy of “neither confirming nor denying” the presence of nuclear warheads on their warships. By 1983 public opinion had swung 72% in favour of banning warship visits. Not long after, Dewes enrolled in a peace studies program at the
University of Bradford, whilst juggling motherhood with a teaching career and a number of official positions and voluntary roles. She is married to Robert Green, a former
British Royal Navy commander, who partners with her in advocating for peace, disarmament and against nuclear proliferation. The senior journalist, Mike Crean, in an interview with Dewes after her New Year Honour, explored that idea that the strength of her feelings came from her ancestors; for she had only recently found out that not only did her paternal great-grandmother work for peace among the northern Hawkes Bay Māori in 1870, but also her maternal grandparents had campaigned for
temperance and
women's suffrage in the late 19th century. == Campaigns and professional achievements ==