Coates is replacing list MP Kevin Hague who resigned to become head of Forest and Bird.
An economist and one of the leading Auckland activists against the Trans Pacific Partnership, Coates has been appointed Green spokesman for trade, which he touched on in his speech.
"We need better alternatives to these unbalanced treaties," he said. "Small gains through tariff reductions should not be used as a Trojan horse to allow foreign investors to sue governments."
Coates is also spokesman for internal affairs, commerce and consumer affairs, ministerial services and overseas development assistance.
He spent 20 years abroad and worked for some time with the WWF (the World Wide Fund for Nature), and the World Development Movement.
Back in New Zealand he worked for Oxfam and has been involved in climate change and anti-poverty campaigns. Most recently he worked at developing a sustainability programme at Auckland University Business School.
In 2014 he stood in Mt Roskill for the Green Party.
"The golden thread through my career has been a passion for sustainability and social justice.
"I have achieved change from outside politics and now I want to achieve within the political system."
He said inequality had risen to unprecedented levels.
"The rich are getting richer while a growing precariat struggle with low incomes and high housing costs.
"People are feeling alienated from politics and from the power structures they see as unaccountable.
"Our Government has been tinkering at the edges, doing just enough to paper over the emerging crises, desperately trying to maintain the illusion that everything is fine.
"All headlines and no substance," he said.
"We could be coming together around a goal to end poverty and deprivation.
"This is target New Zealand has signed up to as part of the UN Sustainable Development Goals.
"We need to make it a reality in New Zealand."
