"As far as I'm concerned, I'm ruling a line under it and we're moving on from here.''
He said he was "dismayed'' to see Mr Cunliffe acting disloyally.
"I don't feel I can trust him and as a result of that I have demoted him to the backbenches and we are moving on from there.''
Mr Shearer said at Tuesday's caucus meeting 100 per cent of the MPs endorsed his leadership.
"That's about as good as it gets.''
Labour MPs have been gagged from speaking about Mr Cunliffe's punishment, with Mr Shearer appointing himself as the sole spokesperson on the issue.
But there has been anger from some of Mr Cunliffe supporters on Facebook and the Standard blog as well as calls for MPs who had publicly criticised Mr Cunliffe to also be punished.
One of Mr Cunliffe's New Lynn electorate committee members, Greg Presland, said the committee had concerns about the demotion and the criticisms by other MPs, such as the chief whip Chris Hipkins.