Ardern has enjoyed a remarkable surge in popularity since taking over as opposition leader last month. The 37-year-old has been greeted like a rock star at large rallies and has generated plenty of excitement among her fans.
English, 55, has run a more low-key campaign, highlighting his experience and the economic growth the country has enjoyed over recent years. He's promising tax cuts for most workers. Opinion polls indicate there has been a swing back to English in the waning days of the campaign after Ardern had all the early momentum.
Voting continues until 7 p.m. local time with first results expected about 90 minutes later. Figures released by New Zealand election authorities show a record 1.2 million people chose to cast their votes before election day.
That equates to about half of all the votes likely to be cast in the nation of just under 5 million people. Election authorities have been making it easier for people to cast early votes, which they can do at certain polling stations up to two weeks before the election.
It also means that New Zealanders might find out the results quicker, because early votes are counted before the polls close Sitting on his campaign bus as it rattled through some of New Zealand's struggling smaller towns on Thursday, English said Ardern had taken the nation by surprise and made him question himself.
"It tests your faith in your product and your faith in your approach," English said during a rare quiet moment between the frequent stops at cafes and main streets. English enjoyed those low-key meet-and-greets, a contrast to the larger rallies Ardern held.
English said the polls have been volatile, and small changes could have a big effect on the outcome. "People have been changing their views very quickly, and I think the polls have reflected that," says English. "You've really taken what normally takes two to three years in a political cycle and telescoped it into six weeks."
Ardern laughed when asked if she ever expected to do as well as she has so far. "You know what, I really didn't have any time to set any expectations," she said. "It was just hit the ground running, and run a campaign that was good enough to win."
At stake for both candidates is how to capitalize on New Zealand's growing economy. English said people should stay the course after his government set the country on a path toward increasing prosperity. Ardern said she wants to build thousands of affordable homes to combat runaway house prices, spend more money on health care and education, and clean up polluted waterways.
At a rally at an Auckland mall, excited fans asked Ardern to take selfie after selfie with them. "I love her dynamism, her freshness, her energy and her honesty," said Susie Powell, who attended the rally.
English said he thinks the televised debates between the two candidates helped swing the momentum his way, as people focused more on the issues and how policy changes would affect them. But his opponent accused him of scaremongering over her plans for taxes and the economy.
"Certainly it's been somewhat frustrating dealing with their negative campaign," Ardern said. "But, from what I've seen, this is an election that's going to come down to turnout." Ardern is hoping that if younger voters turn out in big numbers, it could help swing the election her way.
English's campaigning went better than many expected, including himself. The former finance minister was seen by many as more of a numbers guy than a schmoozer. But he said he'd been surprised at how much he'd enjoyed all the handshaking. And New Zealand, with a population of just under 5 million, is a small enough place that he met some old colleagues by chance.
"When you've been in government, there's always some proportion of people who aren't happy with the things you've done, or haven't done," English said. "But I've found people at least polite. Very occasionally rude, but usually at least polite and often very warm."