Dewes grew up in a "conservative family" as one of eight siblings.^[4] Her father was a veterinary surgeon.

She was head prefect at Hamilton Girls' High School.^[5] 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.^[6] Playing that song to her pupils inspired her to become involved in the peace movement.^[7] She joined a non-violent waterborne protest group called the Peace Squadron, aimed at preventing armed US warships from visiting Auckland Harbour.^[1][4][6]

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.^[8] By 1983 public opinion had swung 72% in favour of banning warship visits.^[9]

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.^[4]

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.^[10]

The senior journalist, Mike Crean, in an interview with Dewes after her New Year Honour,^[4] 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.^[4]

Dewes successfully lobbied for the world's first national nuclear-free laws, known as the New Zealand Nuclear Free Zone, Disarmament, and Arms Control Act 1987.^[1][11] In 1988, she was part of a New Zealand Government delegation to the United Nations Special Session on Disarmament III, and while in New York, marched to promote New Zealand's nuclear-free zone and Nuclear Free New Zealand / Aotearoa.^[12]

She played a key role in the World Court Project that led to the 1996 historic judgement by the International Court of Justice, in The Hague, which unanimously ruled that a threat to use and the use of nuclear weapons is generally illegal according to international law.^[1][4][13] She subsequently completed her doctorate at the University of New England in Australia, with a thesis entitled The World Court Project: The Evolution and Impact of an Effective Citizens' Movement.^[14][15]

Along with her husband, she is co-founder and co-director of the Disarmament & Security Centre (DSC), which they established at their home in Riccarton. She was also a director of the South Island Regional Office of the Aotearoa / New Zealand Peace Foundation^[16] and became Vice President of the International Peace Bureau in 1997.^[6][15] As a member of the Government's Pacific Advisory Committee, she championed issues important to small Pacific Island states, including the knock-on effects of past nuclear detonations at Moruroa and Fangataufu atolls.^[11] She lectured in Peace Studies at the University of Canterbury from 1986 to 1997, where she is an Adjunct Senior Fellow at the College of Arts.^[11][15]

From 2008 to 2012, Dewes was a member of the United Nations Secretary-General's advisory board on Disarmament Matters.^[17] She and her husband were negotiators on the first legally-binding international treaty to comprehensively prohibit nuclear weapons.^[2]

In 2019, Dewes donated the archives of the Disarmament & Security Centre to the Macmillan Brown Library at the University of Canterbury.^[18] In the same year she and her husband were mentioned as potential Nobel Peace Prize contenders.^[19][20]

• http://www.disarmsecure.org/