Afterward he told the Herald he was confident caucus would handle it smoothly, saying all were aware of the danger and wanted to win in 2020. He would not give his own preference, saying it was up to caucus.
Qualities the leader would need included "self-belief and the ability to see the longer view, beyond the necessary focus on the day to day," he said.
English's announcement caught many of his MPs by surprise. While many had believed he would leave prior to 2020, there was no warning it would be this early.
English said he had made up his mind over the Christmas period and decided to go quickly rather than wait.
"I thought that it would be too difficult to do the job properly if I'd half left the building. That wouldn't help the party, it would probably end up creating more speculation about leadership or whatever.
"So I thought that once I'd decided for myself then I was better to go."
English said after the caucus retreat in Tauranga last week he felt confident National was in good shape and would be "strong and competitive" through to 2020.
He had been tempted to stay on to fight the 2020 election but decided to follow the lead of former leader Sir John Key and go while it was still his choice.
"At some point you have to let go. It's inevitable in this business. And if you have the opportunity to choose the time at which you do that, then you should take that opportunity."
English's resignation is effectively the end of the Key-era, and English said he did talk to Key about his decision.
"It seemed to me that what would work best for the party is the same as what worked best in the transition from John Key.
"That is a leader who had strong support in caucus making their own decision to go and then a positive environment to choose a new leader because they want to – not just because their last one ran out."
English's departure prompted messages of regard from across Parliament, including Prime Minister Ardern and Green leader James Shaw.
The exception was NZ First leader Winston Peters, who sent English into Opposition after opting to go into government with Labour.
Peters said whoever replaced English would be unsuccessful and he had no kind words for English.
"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."
English laughed it off.
"Winston Peters is in a world of his own. I don't pretend to understand him."
English said he does intend to work after leaving politics, joking he had always wanted to drive a silage truck.
