Ms Harre told the New Zealand Herald that Mr Key's resignation as Prime Minister would give people on the left confidence that next year's general election was "a whole lot more winnable".
"It will make an enormous difference. National had fully linked itself to him in terms of their public identity. I think his star was fading but he was still the strongest candidate they had for leader by a country mile.
"And his departure definitely gives people on the left confidence that the election just became a whole lot more winnable."
Ms Harre, who is co-owner of the Ika seafood restaurant in Auckland's Eden Terrace, joined Labour at 15 but quit in 1989 after disagreeing with the "Rogernomics" economic reforms advanced under Finance Minister Roger Douglas.
She has been a member of the NewLabourParty/Alliance, the Green Party and was leader of the Internet Party, which was bankrolled by internet mogul Kim Dotcom and entered an alliance with the Mana Party at the 2014 election. That was a failure - Internet-Mana gained only 1.4% of the vote.
Asked if, given her role with the Internet Party, she expected resistance from people within Labour, Ms Harre said the party was a broad church.
"We came through the hostility there was between Labour and the Alliance, we proved that if you work together you can win. There really isn't time for people to dwell on smaller battles, there's a much bigger battle to be fought here."
Ms Harre said she had talked to a number of people in the past year or two about how to advance progressive politics, particularly with younger people and those who had been involved with the Alliance and in trade unions.
"In advising others I realised my strong view was that rebuilding confidence and support for Labour is really the critical factor for electing a progressive government next year."
She said she had a similar background to Labour leader Andrew Little, both being lawyers who had worked for trade unions.
Mr Little was leading the EPMU and Ms Harre was general secretary at the National Distribution Union when the two unions represented 520 supply chain workers at Progressive Enterprises during a weeks-long lock-out in 2006.
"Through those sort of battles you really do get to know and appreciate people. Or not. And I have a lot of confidence and trust in him."
Ms Harre said she had talked to people within Labour about standing in next year's election, and had received encouragement from some to do so.
"I feel that I can make a really strong contribution, I think I've got excellent experience, a lot of policy and historical knowledge, which would be very helpful. I can demonstrate through my achievements a pretty good record of effectiveness in Parliament. I think it's a place that I can make a really strong contribution."
Labour's general secretary Andrew Kirton said Ms Harre shared Labour's values and had a wealth of experience.
"She is great talent and she is welcome. She will have to go through the same process as everyone else. If she wants to stand in a seat she will have to put herself forward in front of local members and make the case."
Mr Kirton said some people within Labour may be wary of Harre rejoining given her recent role with the Internet Party, but said most people were likely to be supportive.
"I think most people are pretty relaxed about that. They see her as someone with a long history supporting the left and standing up for people."
