"The next Secretary-General must have the courage, experience and skills necessary to lead this organisation, to keep it relevant and responsive.
"I've worked across the political divide from Helen Clark for years and I know her to be a natural leader.
"She rallies people together to find the common ground, even when the issues are difficult and the differences vast.
"She gets things done. We think it's time for a Secretary-General like Helen Clark."
Key told reporters after his speech that he had introduced Helen Clark to US president Barack Obama and later to the British Prime Minister Theresa May during a reception at the UN.
"We were all there, the three of us," he said about the Obama meeting."
They had a bit of a chat and then she wandered off.
"She was working the room pretty hard."
Key said Obama "absolutely knows who Helen Clark is" and he was aware of New Zealand's campaign for her.
"We've raised it with him in the last few days. We've raised it with him over the last few months.
"Let's wait and see how it all plays out but we are doing everything we can."
Key introduced her to Theresa May as well - the only P5 leader Clark had never met before.
"I took her over to Theresa and said 'look you haven't met, you guys should get to know each other.'"
He acknowledged it was unusual to be electioneering from the podium at the UN general assembly.
"We have backed her from the start and we are going to back her right until the finish."
The contest was reaching a critical time with the last of the indicative votes on the Security Council next week and the first showing potential vetoes of the P5 members the following week.
Clark was in the General assembly to hear Key's speech.
She told the Herald this week that she had previously shaken Obama's hand but that candidates for the Secretary General post formally presented their cases to foreign ministers, not leaders.
Key is chairing a special meeting of the Security Council tomorrow on Syria - New Zealand is chairing the council in September - and he was critical of previous inaction of the council on the matter.
"The internal politics within the Council and the sheer complexity of the Syria crisis have obstructed a unified Council response," he said in his speech.
"But we believe that no matter how difficult and sensitive the issues, the Council cannot watch the situation go from bad to worse for the Syrian people. "
A meeting by itself would not stop the conflict, he said.
But after more than five years of brutal fighting and horrific humanitarian suffering, it was not credible for leaders to come to New York and not address the stark realities driving the conflict.
We believe that no matter how difficult and sensitive the issues, the Council cannot watch the situation go from bad to worse for the Syrian people.
"We hope tomorrow's meeting will provide an opportunity for Council leaders to take stock of developments, examine the fundamental issues at the heart of the conflict, and discuss how we can move towards a sustainable political solution.
"We also hope the meeting will be a chance for the Council to put its weight behind the ceasefire agreement reached between the United States and Russia.
"That agreement is the best chance we have had in some time to stop the fighting, get aid to those who need it, and get back on track for a political resolution to the crisis."
Like US President Barack Obama's speech this morning, Key talked about the advantages of an open trade system and the Trans-Pacific Partnership in particular.
"In the Asia Pacific region, closer economic integration through trade agreements like the Trans-Pacific Partnership and Regional Comprehensive Economic Partnership will also bring us closer together," he said.
"We must guard against creeping protectionism, and be willing to make the case for more economic engagement clearly.
"We cannot turn inwards. We can't allow fear, or narrow domestic interests to turn us away from an open global trading system, which has lifted millions out of poverty. Countries that close their borders can't do business."
New Zealand has long argued for reform of the United Nations and against the veto and today was no different.
Key said the use of the veto by the Permanent Five members of the Security Council, the threat of the veto and the exploitation of the veto was well beyond what the founders of the United Nations envisaged.
"The time for moving forward on Security Council reform is now. We will be pushing more than ever for action in the General Assembly to make the Council better reflect today's realities and needs."
New Zealand's two-year term on the Security Council ends in December.
