She felt that to change a party, a leader had to be prepared to offend some people, and Mr Shearer had not taken clear-cut stances on policy he disagreed with fearing it would be unpopular with some members.
Labour electorate chair for New Lynn Greg Presland said the resignation came out of the blue because he felt Mr Shearer was improving.
He emphasised that the Labour leader was up against a formidable opponent in Prime Minister John Key.
Mr Presland predicted that a leadership battle would re-energise Labour, in a similar way to the leadership change in the Australian Labour Party last month.
MPs also expressed surprise at the suddenness of the resignation.
Maori Party co-leader Te Ururoa Flavell said he was shocked by Mr Shearer's decision but added that it was probably inevitable. "I just feel sorry for the guy. Bearing in mind that most people that come in here come with the very best intentions and he obviously came back in the belief he could give something to the Labour Party and to Parliament. He did his best, and that's all you can ever ask for a person in here."
The Mana Party's Hone Harawira said he always found Mr Shearer to be "very, very friendly and very open".
"I think that was probably his downfall. Nice guys don't last long in this game."