At a press conference in the Labour caucus room at Parliament this afternoon, Mr Robertson said he would take some time to reflect on how he was beaten to the top job.
"As we went around I felt I was getting strong levels of support. I felt that the husting meetings went well. But, look, this is the process we've got ... and I support the outcome of it."
The Wellington Central MP and deputy leader said he was aware of the challenge he faced in winning the leadership but he had remained optimistic.
"I have a lower profile than David, I haven't been in politics for as long ... but I gave it my best shot."
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.
"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."
Mr Robertson thanked the other leadership candidate Shane Jones and said he hoped Mr Jones was given a senior role.
Mr Robertson said the three-week leadership campaign had been a "historic process" which had revitalised the Labour Party.
- By Isaac Davison of the NZ Herald
