A lot of people would be on holiday in August, and September would be busy with many world leaders visiting the UN.
"The general view is that it probably wouldn't be determined before October."
Ban Ki-moon's second five-year term finishes on December 31.
The UN rules require the Security Council to recommend a person to the General Assembly for appointment.
Helen Clark was an early bookmaker's favourite from among the first nine candidates but two strong contenders have since entered the contest, Miroslav Lajcak from Slovakia and Susana Malcorra from Argentina.
Helen Clark said she had been approached about putting herself forward for the vacancy in 2006 but as Prime Minister of New Zealand at the time, she had a job to do at home.
"As it happens, now I am available."
As well as her nine years as Prime Minister, the past seven years as head of the United Nations Development Programme had given her experience in handling very difficult situations, including in Africa.
"With UNDP you have incredible exposure to the needs and interests of developing countries.
"I don't think any other candidate would have had the contact with Africa and its issues which I have had, which is where the big concentration of poverty in our world remains.
"I put myself forward knowing that I have the background to do this."
She said she had made the United Nations Development Agency the most transparent aid organisation in the world - rated as such in the Aid Transparency Index for the past two years - and she wanted to do the same for the United Nations.
"What we did was turn the place inside out, so instead of refusing to give anybody any information, we put everything on the internet, so you can find out anything about any project the UNDP has anything to do with.
"I think the UN proper needs a dose of transparency through it."
The willingness to engage through social media was part of that, too.
Helen Clark declined to give a view on the British decision to leave the EU saying:
"I wish I was free to say what I really thought because as an international diplomat you really can't.
"The people have spoken in Britain, not by a big margin, so it is going to be a very sensitive matter for Britain's leaders and Parliament to navigate through.
"Now minds will have to turn as to how to carry out that decision without it being highly disruptive on people's living standards, on the British economy and society."
She was well acquainted with New Zealand companies which had located in Britain as a base for exporting into Europe.
"I can tell you all those companies right now are going to be saying 'am I in the right place?"'
She had a nephew farming in New Zealand who would be wondering what it meant for the price of meat and young Kiwis would be wondering what it meant for holiday schemes.
"I really hope these avenues will stay open for New Zealanders."
