He said he not considered the position of deputy leader under Mr Cunliffe, but would have a conversation with the new leader soon. In the meantime, he would step down as deputy leader.
Asked whether he would rule out a leadership bid in future, he said: "I'm 41 years old. Once David Cunliffe has done three or four terms as Prime Minister, you never know, it could be my term then."
He added: "I believe that David is going to be a great leader and an outstanding Prime Minister next year."
He said the new leader had his 100 per cent loyalty and "my total commitment to work over the next year to see him become Prime Minister".
Mr Robertson received the largest slice of the caucus vote, with 47 per cent of MPs backing him.
Asked whether he was concerned that less than a third of the caucus voted for the new leader, Mr Robertson said: "I don't believe so at all... We all understood that this process would lead to a person being elected on the basis of whether they could garner support from right across the Labour Party. David has quite clearly demonstrated that he has done that."
He thanked the other leadership candidate Shane Jones and said he hoped he was given a senior role.
Mr Robertson said that the three-week leadership campaign had been a "historic process" which had revitalised the Labour Party.