"I can empathise with his reasons. Politics requires much sacrifice. We may all be politicians, but not all our lives are politics.
"The Prime Minister has served New Zealand through times of considerable global instability, and will leave politics proud of his achievements. I wish him and his family the best for the future.
Little said "labour is ready and willing to contest the 2017 general election."
Fellow National MP Steven Joyce has called Key's decision "courageous".
New Zealand First Leader Winston Peters said of the resignation: "The fact is that the economy is not in the healthy state that the Prime Minister has for so long claimed, and there are other issues which have caused this decision as well.
"The New Zealand public should have been informed of this a long time ago.
"Clearly the Prime Minister does not believe the superficial polls any longer.
"Contrary to certain perceptions the Prime Minister and his Finance Minister are unable to muddy the waters anymore."
Green Party co-leader Metiria Turei has said that while she fought against Key's politics, she "always supported his right to be a dad & a husband first".
In a statement, co-leader James Shaw also thanked the Prime Minister for his services on behalf of the party and its MPs.
"No matter your political allegiance, you have to respect someone who chooses to make the personal sacrifices required to be our country's Prime Minister.
"I would like to pass along our best wishes to him for whatever his future holds, and to his wife, Bronagh, and children Stephie and Max as well, who I'm sure have made many sacrifices of their own.
"Being the leader of a major political party, and indeed the country, is not an easy job; Mr Key should be applauded for his commitment to public service and to New Zealand," said Shaw.
Asked whether the resignation would boost the Opposition, Shaw said his party wanted to use today to acknowledge the Prime Minister for eight years of service.
"It clearly changes things for next year [when an election will be held]. But today we just want to say - eight years, thanks very much for your service.
"It is a very tough job, and I don't think it matters what side of the political spectrum you are...you have to acknowledge that it is very difficult, very tough on families."
On Key's legacy, Shaw said Key steered the country through the Christchurch earthquake and the global financial crisis - a "double whammy" - and managed to reassure New Zealanders during that time.
"Our criticism of his Government has always been that it is very short-termist and often focussed on looking like they are trying to solve the great challenges of our time without actually making any of the big calls."
ACT Leader David Seymour also offered his support of the PM, congratulating him on his "noble" resignation.
"Under John's leadership, the Government has steadfastly maintained New Zealand's policy settings. As a result, we remain at the top of almost every international league table for good policy settings. In the long term, all Prime Ministers are judged for the policies they leave behind, and John will be judged well.
"It is a reality of MMP that ACT has played a vital role helping John to become and remain Prime Minister. He thanked me for that this morning. I'd like to thank him on behalf of ACT and its previous leaders for the constructive way we've worked together over the past eight years.
"We also extend our warmest regards to Bronagh as the Keys get their lives back after a decade of service to the country."
Tamati Coffey, the Labour Party candidate for the Waiariki seat in next year's election, said his reaction on hearing the news was like the rest of New Zealand - one of shock.
National MP for the Bay of Plenty Todd Muller said the news was a complete "surprise and shock".
Key's efforts have also been recognised across the ditch with former Prime Minister of Australia Tony Abbott having posted to Twitter.
Kiwis have taken to social media to express their shock at the news of Key's resignation.
"Was talking about John Key stepping down from PM with some friends last night but wouldn't have picked an announcement 18 hours later," wrote another.
Others are calling his resignation "fishy" and are seeking answers.
Councillor for Auckland's North Shore Richard Hills responded to the news on Facebook saying this year is all about "shock politics".
John Key resigned? This year is all about shock politics. That's a lot of damage you've done Michael Wood 😳
Posted by Richard Hills on Sunday, 4 December 2016
Labour MP Michael Woods won the Mt Roskill byelection over National's Parmjeet Parmar in a smash victory on Saturday.
On Twitter, people compared the National party leader's resignation to the announcement, made within hours of Italian Prime Minister Matteo Renzi's similarly unexpected resignation.
"Two Prime Ministerial resignations in one night - Matteo Renzi and John Key. Who's next," one person wrote."
Some questioned where the party would go to next, now that the unprecedentedly popular Prime Minister was stepping down.
"I've been saying for a while, and thinking for even longer, that National's only problem at the moment is who can replace John Key."
"John Key has just thrown the National Party under a bus. Who will pull it out before the wheels crush it. Bill English is the all-time loser," wrote one skeptic of Key's endorsement.
And a few people took the moment to joke around.
"Prime minister stepping down mid-term. Who are we, Australia? #johnkey #nzpol," said one person, a dig at Australia's revolving cast of Prime Ministers in recent years.
Many have taken to Twitter suggesting Key's DJ son Max Key replace his father as Prime Minister.
