Comments on the left-leaning blog The Standard called for the party to unite behind the new leadership. But several users had previously endorsed Mr Cunliffe and they reacted strongly.
"Parliament is going to be a slaughterhouse with Labour on the block. Shearer will be eviscerated!" wrote Hami Shearlie. David H said National would have Mr Shearer "for breakfast". But Mike Smith wrote that Mr Shearer had the track record to lead.
The Council of Trade Unions and the Service and Food Workers Union applauded Mr Shearer's immediate call for a cross-party on the ministerial inquiry into poverty, a group established in a deal between the Maori and National parties.
Political commentator Bryce Edwards said Mr Shearer was a bold choice.
"Shearer probably has until 2013 to revive Labour's fortunes. If he fails, by that time Robertson will be ready to seize his chance."
National member and Kiwiblogger David Farrar called Mr Shearer a "decent man", but questioned the wisdom of choosing a deputy who had his own leadership ambitions.