Mr Shearer's first action after his win was to pay tribute to Mr Cunliffe and Ms Mahuta.
Mr Shearer is expected to ensure any bitterness from the contest against Mr Cunliffe does not turn into division by offering front-bench roles to him and Ms Mahuta, and promotions to some other supporters.
It is a move similar to that taken by Helen Clark when her leadership was challenged in the 1990s.
He will spend the next week working out his front bench and portfolios to ensure the Labour team is in place when Parliament opens on Tuesday.
Mr Shearer would not discuss individual choices for his front bench, saying he had not yet spoken to his colleagues to assess their preferences.
However, he said Mr Cunliffe was highly talented and he expected him to keep a high position.
Others likely to make it to the front bench include Jacinda Ardern as Labour's highest ranked woman, and Shane Jones.
Su'a William Sio and Lianne Dalziel are other Cunliffe supporters who are likely to get promotion, although not to the front bench.
Mr Jones is understood to be interested in an economic role, but said yesterday it was up to Mr Shearer to decide what he took. He said it was critical Labour had a strong front bench, both in terms of ideas and performance in Parliament.
Mr Shearer got one last taste of fun last night - he had promised to take his 14-year-old son to the Foo Fighters concert in Auckland and refused to break the commitment, saying he was also a fan with about four albums.
He is already facing speculation he risks a leadership challenge after two years if he does not improve Labour's fortunes. He said he had no doubts about the expectations.
"I think I've got to prove myself from day one. I've got to make a mark and make sure the Labour Party makes a mark. I have no doubts about that."
Former Labour leader Phil Goff said Mr Shearer's lack of "baggage" because of his short time in Parliament made his job of rebuilding the party easier. He did not want to be on the front bench, "but I'm not about to cut back to half-speed in Parliament."
Mr Goff said he would take a hands-off approach to the new leadership, saying he would be on hand if his views were sought but would not interfere.
Despite the defeat, most supporters of Mr Cunliffe were upbeat about the result, saying they would fall in behind the new leadership team.
Lianne Dalziel said there was a strong feeling they had to back Mr Shearer. Although they had favoured Mr Cunliffe, it was not an "anti-Shearer" vote, and there was no antipathy toward him.
Mr Shearer will receive a pay rise of $116,000, from a basic MP's salary of $141,800 to $257,800. He will also have the services of a chauffeur driven car.
