Prime Minister Jacinda Ardern posted: "Just heard the news that Bill English has decided to stand down. Bill has made a huge contribution through his time in office and to politics generally. I admire those who serve NZ in this place, and Bill did for a long time, and he did it well. My best wishes."
Ardern said in a statement her outgoing political opponent garnered the respect of many.
She said National leader Bill English had clear convictions, and a genuine concern for the well-being of New Zealanders.
Ardern said public service had a huge impact on a politician's family, and English's wife and children have made great sacrifices for his job. She wished them and the outgoing politician the best for the future.
National's Deputy Leader Paula Bennett said Bill English would be terribly missed.
She said he had been an incredible leader.
Bennett wouldn't be drawn on whether she would be vying for the leadership - or comment on the future of her role as deputy.
Former Prime Minister John Key also wished English well.
"Like so many Kiwis I am saddened to hear my close friend Bill English is leaving Parliament.
"Bill has given remarkable service to a party and a country he loves.
"His dry wit outstanding economic leadership and rolled Rs will be missed.
"Enjoy your new life, mate."
National Party president Peter Goodfellow said English could be proud of a "long and distinguished" career in politics.
"His prowess as Finance Minister was instrumental in getting New Zealand through the Global Financial Crisis and ensuring we were one of the first developed countries to get our books back in surplus, while still maintaining support for essential services and for vulnerable New Zealanders."
Goodfellow said National was stronger because of English's leadership.
Other reaction
New Zealand First leader Winston Peters said English had been the steady hand behind John Key, but that couldn't be regarded as successful.
"He's a person that stabilised things under a very flighty leader called John Key, he did the hard yards, so to speak, but in the end, if you look at the concretisation of wealth in so fewer hands in that period of time, this cannot be regarded as a period of success economically."
Peters said he did not think anyone in the current National caucus could win the next election: "They can't debate. They may raise all the money in the world but money won't get you home next time.
"Whoever the next leader is, they're not going to be successful. That I can guarantee you."
Asked if he had any kind words for English, Peters said: "I'll leave those nice words to you.
"I don't think he wants to know about sympathy or otherwise. He's a big, grown, aged man, so to speak, and he's always known how rough this game is because he's been part of it, and [been] in fact inside coups, and sooner or later it will come to haunt you.
"That's what's happened here today."
Green Party co-leader James Shaw wished English well, and praised him for his commitment and perseverance over a 27-year parliamentary career.
"On behalf of the Green Party, I'd like to acknowledge Bill English and his 27 years of public service," co-leader James Shaw.
"Mr English has held many roles during that time – constituent MP, party leader, minister and Prime Minister – and should be congratulated for both his commitment and perseverance.
"Anybody who's involved in politics is aware of the great personal sacrifice that is required to do this job, for both the politician and their loved ones. For that reason, I'd also like to acknowledge Mary English and the entire English family.
"On behalf of the Green Party, I'd like to wish Mr English and his family all the best for the future."
Maureen Pugh is next on National's list and is expected to return to Parliament as an MP.
A former Westland mayor, Pugh served as an MP in the previous term when she replaced senior minister Tim Groser, who left to take up the role as Washington ambassador.
Pugh thought she had made it back to Parliament on election night last year but lost her spot after the special votes were counted.
Pugh attended last week's National Party caucus meeting in Tauranga and said she would take up the chance to return to Parliament it a position came up.
ACT leader David Seymour said English would be a loss to Parliament. "Bill English is one of those rare MPs who comes to Parliament to studiously improve our country's public policy," he said.
The National Party would have been absolutely stuffed without Bill English's knowledge, sensitivity and reasonableness, former Whanganui MP Chester Borrows said.
The current government's extra $800 million surplus is virtually all down to English, Borrows, a National Party stalwart, said.
At the same time English hoped to change social welfare provision by his social investment approach - an approach that puts funding in place for the most vulnerable in order to save on prison and welfare costs later.
"His argument wasn't just around the money. It was about the moral responsibility too," Borrows said.
There's nobody quite like him in the National Party at present, he added.
""He is almost entirely without ego. It was never about Bill. It was always about New Zealand and the National Party."
Current Whanganui MP Harete Hipango said Mr English deserved recognition and respect and wouldn't speculate on who would become the next leader.
"We are still a strong, united caucus. It's just certain that we are moving into another phase," she said.
Coromandel's National MP Scott Simpson said English's resignation was a surprise, particularly after the support expressed by MPs at the Tauranga caucus retreat.
"I am naturally saddened at his announcement. But I can understand that after 28 years in Parliament he wants more time with his family.
"He and Nick Smith are the last two of the original Bolger 'intake' from 1990. I have worked closely with him over the years.''
He said English would step down on February 27 and a new leader would be elected the same day.
"Contenders will make a pitch to caucus in the meantime. And no, I won't be putting my name forward!"
Left wing political commentator Josie Pagani said English's resignation was a big loss for the National Party.
Labour MPs walked out of their own caucus to be collectively surprised at the news Bill English was stepping down.
Minister Megan Woods says they have a party policy of not looking at their phones during meetings so the news was quite a shock.
Rotorua MP Todd McClay said there had been no discussion about English's potential departure at last week's caucus meeting in Tauranga.
McClay heard the news from Rotorua, after flight issues prevented him travelling to Wellington.
McClay declined to rule out a leadership bid or discuss any possible candidates.
"All I'll say is that he's only made the decision today," he said. "This is Bill English's day."
However, McClay said he believed the leadership selection process would be "constructive and calm".
Said satirist Toby Manhire: "woah. Bill English is pregnant."
Several overseas news agencies focused on the announcement being made after the election loss.
Sky News Australia tweeted: "The former Prime Minister's resignation comes less than six months after his election loss to current leader @jacindaardern."
Noted SBS News: "Bill English quits as New Zealand's National Party leader after election loss."
Reported The Guardian: "The former New Zealand prime minister Bill English has resigned as leader of the opposition less than six months after being defeated by Labour's "stardust" Jacinda Ardern."
Former Maori Party MP Marama Fox said: "I have great respect for Bill @RtHonBEnglish and enjoyed getting to know him during the brief time I worked with him. I believe his legacy has been to support and help usher in Whanau Ora with Dame Tariana and, bringing independent Maori voice to the seat of Government."
The NZ Young Nats tweeted: "Bill, you got up again. Thanks to you, your dedication, and your integrity, so will National. #comebackkid @RtHonBEnglish
National Party MP for Taranaki/King Country Barbara Kuriger said she was "in awe" of what Bill English achieved and hoped the caucus could continue that good work, especially in the financial field and social investment.
"Bill used to say if you can measure something, you can manage it and you will go a lot further.
His expertise, working alongside his leader John Key, got us through the Global Financial Crisis and made a huge impact on our ability to recover from the Christchurch Earthquake.
Bill made a huge effort in social investment and I would love to see us continue that work and implement some of his programmes."
Green Party MP JulieAnneGenter posted: "I didn't usually agree with @RtHonBEnglish on policy, but I always respected that he is open to a thoughtful debate, and that his motivation comes from his values. Best wishes for your next endeavours."
National MP for Hamilton East David Bennett said: "Bill has done nearly 30 years in the job and he has a tremendous degree of support for him within the electorate and also within the party along with his commitment to himself and his family. They have been tremendous servants to New Zealand and we will miss them dearly."
Opined political commentator Bryce Edwards: "Yet another political leader who 'leaves voluntary', but with a big push."
National MP for Hamilton West Tim Macindoe said there was little to suggest English would be making his intentions known.
"I was a little bit stunned, I did not see the announcement coming when we walked into caucus this morning.
"I sat in the meeting this morning where there was just an incredible appreciative and respectfulness towards him and we all know what a great guy he is and what a tremendous job he has done.
"I have total respect and admiration for Bill, I've always been a huge supporter of his and I feel really pleased that he is able to go out with his head held high having made his own decision with the support of his family and the total support of his caucus."
Martyn Bradbury posted on Facebook: "Bill English destabilised as Leader before conference + Chris Bishop having snapchat allegations leaked to stop him running as leader = Judith Collins - release the kraken."
