Locke was in Parliament from 1999 to 2011, and was known as a human rights and anti-war advocate.
His family said he had died peacefully in the early hours this morning after a long illness.
"He will be greatly missed by his partner Michele, his family, friends and colleagues. He kept up his interest and support for the causes he was passionate about to the last.
"He was a man of integrity, courage and kindness who lived his values in every part of his life. He touched many lives in the course of his work in politics and activism."
The son of activists Elsie and Jack Locke, Keith was politically aware from an early age, and was involved in the first anti-nuclear and anti-apartheid marches of the 1960s.
After a Masters degree at the University of Alberta in Canada, he returned to New Zealand and left academia to edit a fortnightly newspaper for the Socialist Action League, a union he had joined as a meatworker then railway workshop employee.
He joined NewLabour in 1989, which later became part of the Alliance party, and split off into the Greens when they broke apart from Alliance in 1997, entering Parliament as their Foreign Affairs spokesperson in the subsequent election two years later.
While in Parliament, he was a notable critic of New Zealand's involvement in the war in Afghanistan and the Terrorism Suppression Act 2002, and advocated for refugee rights including in the case of Ahmed Zaoui.
He also long advocated for New Zealand to become a republic, putting forward a member's bill which would have led to a referendum on the matter.
Commentators dubbed him variously the 'Backbencher of the Year' in 2002 - an award he reprised from a different outlet in 2010 - as well as the 'Politician of the Year' in 2003, and 'Conscience of the Year' in 2004.
He was appointed a Member of the NZ Order of Merit for services to human rights advocacy in 2021, received NZ Amnesty International's Human Rights Defender award in 2012, and the Federation of Islamic Associations of New Zealand's Harmony Award in 2013.
In a statement on Friday, Green Party co-leaders Marama Davidson and Chlöe Swarbrick said Locke was a dear friend and leading figure in the party's history, who never wavered in holding government and those in positions of authority to account.
"As a colleague and friend, Keith will be keenly missed by the Greens. He has been a shining light for the rights of people and planet. Keith Locke leaves a legacy that his family and all who knew him can be proud of. Moe mai ra e te rangatira," they said.
"From 1999 to 2011, he served our party with distinction and worked extremely hard to advance causes central to our kaupapa," they said.
"Not only did Keith work to defend civil liberties at home, but he was vigilant in highlighting human rights crises in other countries, including the Philippines, East Timor, West Papua and in Latin America.
"We particularly acknowledge his strong and clear opposition to the Iraq War, and his commitment to an independent and principled foreign policy for Aotearoa."
They said his mahi as a fearless defender of civil liberties was exemplified in his efforts to challenge government overreach into citizens' privacy.
"Keith worked very hard to introduce reforms of our country's security intelligence services. While there is much more to be done, the improvements in transparency that have occurred over the past two decades are in large part due to his advocacy and work. We will honour him by ensuring we carry on such work."
