"It was a hard decision because a year ago I was forced to stand down, and we'd crawled back up to 34 per cent. I believe I had the capacity to get back to that and beyond, but there would have to be a fundamental shakeup to address why people aren't viewing Labour as the party that speaks to them."
"There are things that need to be done and I don't think there is acceptance of that amongst much of the party."
He said that was because the party was already absorbed in the leadership contest rather than waiting for the review and analysis that might have helped decide which leader was the best choice for the change needed. "I don't want to be one of those personalities when I don't think tweaking the person alone will address the problems."
He would not say who he planned to back himself, but said he would support whoever won. "When you're down and trying to climb up it's a really tough job. So I have some sympathy for David [Cunliffe] in that respect. He ran a credible campaign."
He said he would stay on in Parliament. "I have no plans of leaving. I want to help in rebuilding. It is the party I belong to and I share its values."
Watch: Labour: Cunliffe quits leadership race
Mr Shearer said his family had said they would support him in whatever he decided to do, although they knew the job was draining. He said there were at least two or three major problems that needed addressing - including fixing the Labour Party brand so it wasn't seen as simply a collection of special interests.
He said the current calls for a female or Maori deputy leader to balance out the white, middle-aged men running as leader was one sign of that obsession. "People are saying we'll have a white bloke for a leader so we need a woman or Maori as deputy. No. We need the most competent person whether that's woman or a man. That speaks a lot about the party."
He said it had lost middle New Zealand. "What happened to the tradies, and the self-employed, the people in offices who just don't see us looking like them anymore?"
Nominations for Labour's leadership close at 5pm on Tuesday - there are now three candidates: David Parker, Andrew Little and Grant Robertson.