Mr Robertson was deputy under David Shearer and won more support from the caucus than Mr Cunliffe did on first preference votes: Mr Cunliffe received 11 votes, Mr Robertson 16 and Shane Jones 7.
People close to Mr Robertson suggested he would not want to put himself forward as deputy without Mr Cunliffe's encouragement.
Referring to the Labour caucus Mr Cunliffe said: "There is a new train leaving the station and I want everybody on it. There will be one set of rules for all. It will be a meritocracy ... any history is history. I have no personal prejudices against any member of caucus."
Asked if he would embrace his enemies as Helen Clark had done in 1996 he said "Yes I will."
Mr Cunliffe traversed a range of issues with ease, confidence and brevity.
"We are going to a war footing," he said.
"We are going to be taking on the Key Government. We are in campaign mode from Day One and we intend to be the Government as soon as that election is called."
He said he would fight on issues, and his style was not to play the person.
Asked about John Key's leadership style he said "I think it sucks and I think it's time to change it."
He said that as a person he liked Mr Key and he had many interesting attributes.
They were backbenchers together in West Auckland before Mr Key became Prime Minister.
"That means I am not fazed by John Key. I believe I have his number and I believe he knows I do."