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Mr English's resignation will take effect on February 27 and he will deliver his valedictory speech on March 1.
Mr Woodhouse was not at the caucus announcement, and learnt the news only a few minutes before the rest of New Zealand.
''As I understand it, a limited number of close colleagues knew about the decision. There was not a hint of it before the announcement.''
At the weekend's two-day caucus in Tauranga, Mr English consistently said he had the support of his caucus colleagues.
Mr Woodhouse, who was a minister when Mr English was prime minister before the 2017 election, said Mr English was very supportive in the early stages of his political career.
When Mr Woodhouse sought the nomination before the 2008 election, Mr English provided mentoring which developed into a bond between the two ''southern men''.
''He is known for his detailed work on issues and I like to know exactly how the policies will play out on the people affected. It is the southern way of doing things.''
While Mr English would be remembered his financial acumen and steering New Zealand through the global financial crisis and Canterbury earthquakes, it was sad he had not been able to fulfil his agenda on social change, Mr Woodhouse said.
Mr English believed strongly in helping people to become not dependent on the state.
''Good on Bill and Mary [his wife] for starting to think about themselves after nearly 30 years. They will leave a legacy which is unparalleled.''
New Clutha-Southland MP Hamish Walker said Mr English had remained loyal to the party and voters by staying on for six months after the election rather than abandoning his supporters on election night.
During the past six months, he had been able to talk to Mr English about the electorate where Mr English had been raised and served as an MP.
Those talks were valuable to a new MP.
Mr Walker said there was a two-week process to take place before caucus would chose its next leader.
Prime Minister Jacinda Ardern paid tribute to Mr English, saying he had worked tirelessly as prime minister, deputy prime minister, finance minister and Opposition leader.
''Very few serve for so long at such a high level, but garner the respect of many.
''He has always stood for what he believes in.''
Mr English was a man of clear convictions who had always had a genuine concern for the well-being of New Zealanders.
Act New Zealand leader David Seymour thanked Mr English for his support of charter schools and his commitment to engaging disadvantaged pupils who had been failed by state schools.
Federated Farmers president Katie Milne said Mr English proved to be a consistently safe pair of hands as finance minister.
''A straight-up boy from Dipton, he is a guy many New Zealanders could relate to.''
Mr English demonstrated he was not just an astute manager of the nation's finances, she said.
He revealed he was a caring leader who really wanted to make a difference for the less well-off.