Phil Goff is the new Labour leader, replacing Helen Clark who stepped down on election night following the party's defeat in the polls.
Labour MPs, meeting at Parliament on Tuesday, gave unanimous support to Mr Goff as leader and Annette King as deputy leader.
David Cunliffe takes the role of finance spokesperson, and former Prime Minister Helen Clark becomes foreign affairs spokesperson.
Phil Goff said he will lead a strong, united and determined opposition, which he said was vital to the effective functioning of a democracy.
He said Labour will support the Government constructively when it believes its actions are consistent with the well-being of New Zealand.
But it will oppose the Government vigorously on policies it views as negative, such as slashing KiwiSaver and Research and Development tax credits or privatising ACC.
Mr Goff paid tribute to former leader Helen Clark and former deputy leader Michael Cullen, who resigned on Sunday, saying their skill, determination and competence are acknowledged even by their critics.
Phil Goff, 55, holds the seat of Mt Roskill in Auckland. He first entered Parliament in 1981, was defeated in 1990 and re-elected in 1993.
He held Defence, Trade, Corrections and Associate Finance portfolios in the last term.
Mr Goff says the Labour Party was defeated in the election because the public wanted change, but that Labour still has a lot to offer.
Right decision - Clark
Before the meeting, Miss Clark said she was comfortable with her decision to resign as Labour leader.
"It's been 15 incredible years and it's been a privilege to do the job, and I think do it to the best of my ability and well.
But I've made the right decision for the Labour Party and for myself and I'm looking forward to giving the new team my total and unconditional support."
New deputy leader, former Police Minister Annette King, is the MP for Rongotai. She entered Parliament in 1984.
"I think that I have the ability to work amongst the caucus, to ensure that we have a strong united team," she said after her selection.
"I'm also not just a smiling face, I can bring to the front bench of the opposition some toughness, some debating skills that we're going to need, because we're determined to be the best opposition we can."
The caucus had been set to meet on Tuesday morning, but that was delayed so Helen Clark, Phil Goff and other Labour MPs could attend Armistice Day commemorations at the Tomb of the Unknown Warrior in Wellington.
Radio New Zealand's political editor says Miss Clark worked extremely hard to unify Labour after the fall-out from the 1984-90 Labour Government, and the party did not want any potential division over the transition to a new leadership.
Miss Clark took over as party leader from Mike Moore in 1993.
She resisted an attempt to persuade her to step aside as the leader in 1996 when Labour was polling about 14%.
David Caygill gave up the deputy leadership at that time, in favour of Dr Cullen.
They lifted Labour's vote to 28% in the election later that year, putting the party in a position to gain three consecutive terms in Government from 1999 until Saturday night.
Earlier on Tuesday incoming Prime Minister John Key described the expected move to appoint Phil Goff and Annette King as "hardly over-inspiring".
"I would have thought they've got some younger people that they might want to bring forward, but in the end it's for them to choose their leadership team."