"Unless we sort out where we want to go as a party together, then actually we don't get anywhere, and that doesn't matter what leader [is at the helm]."
The party had "lost touch with New Zealanders", he said, and needed to focus on what went wrong, rather than blaming David Cunliffe.
"What we need to do is fundamentally not jump to find fall guys, but jump to the fact that we need to reform the party."
He later added: "I do believe that there's a process that needs to happen here and we need to do it very carefully and not jump onto just blaming one person or another."
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He dodged questions on whether he thought Mr Cunliffe was the right man to lead the party, saying a discussion on leadership would need to be taken in the context of everything else the party needed to work on to improve.
While he did not rule out putting himself forward for leader again, he refused to answer the question directly, saying: "That's what I'm not going to talk about today for the simple reason that I don't think it's the right time to be talking about that.
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"We don't want to be dragged down the path where one issue and one issue only [is the focus], when it is a bigger part of the Labour Party that needs to be discussed. As leader I struggled with trying to keep those various parts together, [but] if those parts don't come together and we don't agree on them then no leader, no matter who they are, is going to be able to take us to government."
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Labour needed to have "an honest discussion" about its position on the political landscape, he said.
He believed it needed to sit firmly in the centre, as moving further to the left would be "oblivion". Labour needed to be "wider and broader" to engage more voters, he said.
Mr Shearer pointed to National "reaching across the centre line" in its last budget to adopt traditionally Labour Party policies which appealed to middle voters.