Most Labour MPs, including party leader Phil Goff, distanced themselves from Ms Fenton's outburst, calling it unwise.
But Ms Wall, who is back in Parliament for a second stint as a list MP, said the comments were misinterpreted, and she shared the disappointment at Sir Peter's support for Mr Key - despite admiring his work.
"We would have assumed Sir Peter was a working-class champion," Ms Wall said.
"If you look at what the National Government has done, it has taken workers' rights backwards."
Ms Wall, who is contesting the safe Labour seat of Manurewa, said "personal is political", and Sir Peter could not endorse Mr Key without endorsing his policies.
But retiring Manurewa MP George Hawkins hailed Sir Peter as "a great bloke for South Auckland, absolutely brilliant, has helped many charities."
Labour MP Trevor Mallard said Ms Fenton's comments were unwise.
"I can understand where she comes from. All of us are sensitive about stuff like that, but it's the sort of thing where you take a pile of deep breaths and then let it go."
The Red Alert site has been the medium for Labour apologies before. Dunedin South MP Clare Curran was ridiculed by both ends of the political spectrum after a Red Alert post in which she said the Green Party was encroaching on Labour territory and she had had a "gutsful of the white-anting of Labour".
She soon posted an apology.
Mr Mallard has been the subject of strong criticism as well, most recently for attacking political commentator Bryce Edwards as a being bankrolled by the left and the right to "provide the political commentary which mainly attacks Labour and the Greens from the looney left. The guy makes [Auckland University Maori studies professor] Margaret Mutu look like a well-balanced academic."
Mr Mallard said he did not resile from any comments he made on Red Alert. "It's a place to float things that are slightly out there and say things that you know will be contentious."
The site has been live since 2009 and is seen as a useful window into Labour MPs' thoughts.
