For most of this year New Zealand's next government was presumed to be a foregone conclusion – a done deal, a contest so predictable it was being described as boring.

Now it's a two-horse race too close to call, as the opposition storms from behind with a charismatic young leader who promises radical change.

Associate Professor Grant Duncan, who lectures in political science at Massey University, struggles to find one word to sum up the recent turn of play.

"Unpredictable," he says. "Unprecedented. Exciting!"

"A number of events over the past few months have rather radically changed the situation in unexpected ways."

The governing centre-right National Party had been riding high on the back of an economic surplus, touting its success at pulling the country back from the brink of the Global Financial Crisis.

National was also basking in the afterglow of the John Key effect. One of New Zealand's most popular prime ministers, Key caught the country by surprise when he quit the top job in December less than a year from the election, handing over the reins to deputy Bill English.

English was a safe pair of hands. His eight years as finance minister meant people knew what they were getting.

"Everyone knows he doesn't have that charisma factor, but people know him well enough all the same to kind of feel that he's reliable," Duncan says.

National cruised in the polls. Support consistently registered in the high 40s; the party's popularity barely dented after almost a decade on top.

The centre-left Labour Party struggled for traction. Helen Clark's departure following her election loss in 2008 created a power vacuum and the party cycled through a succession of equally unpopular leaders.

Labour headed into the election under Andrew Little – a former union boss whose biggest failing, perhaps, was that he was even less inspiring than Bill English. Support for Labour languished below 30 per cent.

But then everything changed.

The first plot twist came when Green Party co-leader Metiria Turei took the stage at her party's annual conference in mid-July and revealed she had deliberately lied to the government as a solo mother in the 1990s so she could claim more money on her benefit.

Turei's revelation rocked New Zealand, sparking debate over whether there was enough support for vulnerable members of society.

Support for the Green Party shot up to 15 per cent, further depriving Labour of oxygen as it dipped to a crushing low of 24 per cent.

Labour went into crisis mode. Andrew Little fell on his sword on the first day of August, thrusting the leadership onto his deputy Jacinda Ardern. It was six days after her 37th birthday.

The election was less than eight weeks away.

Ardern promised "relentless positivity" as she introduced herself to New Zealand. Young, attractive, and eloquent, the public listened when she spoke.

Labour shot up in the polls, breaking the 30 per cent barrier, and then, incredibly, breaking the 40 per cent barrier. Support for the Green Party collapsed below 5 per cent as Turei faced scrutiny over her beneficiary background.

"They took a big risk in changing their leadership so close to an election, and that has paid off for them quite dramatically," says Duncan.

Now, with a week to go, Labour and National are neck and neck. Alternate polls show either party pulling ahead, and the result is too close to call.

Ardern's message of change has resonated with voters. Nine years of frustration have bubbled to the surface as National faces questions over its track record.

Shamubeel Eaqub, an independent economist who was formerly the principal economist at the NZ Institute of Economic Research, explains New Zealand hasn't been as rosy as it may seem to an outside observer.

"The economy has really been driven by population growth, with a migration boom to New Zealand," he says.

"Our numbers have also been flattered by the rebuild from the Canterbury earthquake in 2011, which shouldn't be counted as real growth."

Eaqub adds that with average wage inflation running at around 1 per cent each year, it's clear the economic uptick has been lopsided.

"There are a few sectors and regions that are doing really, really well, but we're no different from almost any other country," he says.

"What we're actually experiencing is an uneven and sluggish economic recovery."

Arden is successfully tapping into growing unease over New Zealand's social record. A recent UNICEF report ranked it worst in the developed world for teen suicide, and 34th out of 41 countries in terms of overall child wellbeing.

Housing has become eye-wateringly expensive. A report from the International Monetary Fund last year crowned New Zealand as worst among 30 of the world's richest countries for housing affordability, and homeownership rates are the lowest they've been since the 1950s.

Intensive dairying has left a stain on the nation's clean, green image, with many waterways polluted by effluent run-off. For most Kiwis it's no longer safe to swim in the rivers they enjoyed as children.

National has pledged to make 90 per cent of lakes and rivers swimmable again by 2040 – that's decades away, and simply not good enough for many people.

Ardern hit a nerve at a debate hosted by Fairfax NZ when she berated Bill English for failing the country's youth.

"My generation has been sold down the river by your government," she said, to cheers from the audience.

English's retort was lacklustre; he knew well enough what the younger generation needed, he said, because he had helped to raise it himself. (He has six children.)

His dismissal of Ardern as "stardust" also failed to gain any traction.

Ardern pledges a fresh, compassionate approach to address healthcare, inequality, and the environment. She wants to enshrine poverty reduction targets in legislation, and says each budget should include a child poverty measurement.

She made waves last week when she declared that neoliberalism has failed New Zealand.

"Any expectation that we just simply allow that the market dictate our outcomes for people is where I would want to make sure that we were more interventionist," she told Radio New Zealand.

It may sound like the beginnings of a social revolution akin to the populist wave that propelled Britain out of the European Union and saw Trump elected in the United States, but Duncan hesitates to draw parallels.

"Those anti-establishment, anti-immigration sentiments have not been decisive in this election, so New Zealand can't be put in that basket," he says.

Instead, Duncan adds, it's more accurate to say New Zealand voters are taking a good, hard look at what kind of place they want their country to be.

"There's a real choice between two leaders who are quite different in character," he says. "One is relatively conservative, promising steady as she goes, and the other is young and vibrant, and offering change."

Duncan reflects it's one of the best elections he's seen.

"Regardless of who wins, I think it's been a really healthy campaign debate," he says.

"It's democracy the way that it ought to be."