Mr Shearer said his comments were not simply seeking revenge or a cathartic experience. "It's about making sure we set ourselves up for the future so the new leader doesn't have the same experience I had."
He had been white-anted by Cunliffe's supporters when he was leader and did not want the same thing to happen to the new leader.
Mr Cunliffe and some of his supporters, including Sue Moroney, have said they believed he still would have won the leadership if he had not withdrawn.
Mr Shearer said he did not believe that, but it would have been better for Mr Cunliffe to have stayed in the contest to put to bed that question once and for all.
"Many of his supporters believe he is the True Leader. And I don't believe their ambitions for him will die. They were the people who gave me a pretty difficult time. We could have had a fair contest and whatever happened we would have had a clean line underneath it."
He did not believe he was making matters worse by speaking out. "What happened to me should not happen to the new leader. We need to give the new leader every chance they can get, chances that Phil Goff and I perhaps didn't have.""
Mr Cunliffe has described Mr Shearer's comments as "unfortunate" and Mr Shearer was criticised by Cunliffe loyalist Greg Presland on the Standard blog. Writing under his pseudonym Mickey Savage, Mr Presland said Mr Shearer was simply feeding a "right wing narrative."
"And attacking David Cunliffe and then saying that the attacks need to stop brings to mind a word that starts with "h". I agree with the sentiment that Caucus needs to unite and move on. Note to David Shearer, please do this."
However Mr Shearer said he had been subjected to "undermining and white-anting" by Mr Cunliffe's supporters when he was leader and did not want that to be experienced by the next leader.
"You've got to ask him what his intentions are now. If he wants to stay then what we would want is absolutely for him to get behind the new leader."
He said he would like to think Mr Cunliffe could be trusted "but that's up to him."
Labour MP Clayton Cosgrove pitched in to back Mr Shearer, saying Mr Presland's post this morning only went to prove Mr Shearer's point.
The Standard's Lyn Prentice also recently took down a post criticising Mr Cosgrove at the request of Labour's General Secretary Tim Barnett but has said it would go up again after the leadership contest.
