When the world ends, a teenage girl will save us.

She will beat the bane, the blight, the bogeyman. She will be tested, tormented, troubled. Here she is now: "Come in," grins Greta Gregory. "Can I grab you a coffee?"

In a suburban Auckland far less scary than the one she inhabits on screen, Gregory wears track pants and spectacles and says, "I always feel like high school was just yesterday ... "

Doesn't everybody? And isn't that the point?

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Gregory is the latest version of an enduring trope. The reluctant teenage hero(ine) with a post-apocalyptic postcode. Think Divergent and Hunger Games or even Twilight - except Gregory's getaway van comes with an "I love NZ" keyring.

She's the star of The Cul de Sac, a locally-made youth drama set shortly after the world as we know it has ended, and back for a third season. (The writer-director conceived the idea when he woke up with a large mortgage in a dead end street on the North Shore.)

Gregory is a 25-year-old playing a teenager fighting to keep her siblings safe in a landscape where there are no adults, internet or working cellphones.

"The young adult dystopian sci-fi genre is everywhere," Gregory acknowledges. "Everyone enjoys it. We do have a tween audience - which is what I guess the target demographic is - but I've seen fans out there who are in their 70s, who have said their grandchildren can't watch it because it's too scary!"

The rise of YA or "young adult" film, television and literature, is well-documented. Last year a study found 55 per cent of all YA books were being read by so-called grown-ups. Caroline Kitchener, writing for The Atlantic, noted the universality of "coming of age" stories and the idea that we're all, constantly, coming of age.

"Totally, absolutely," says Gregory. "I feel like you never stop learning as a human being."

Gregory says one appeal of the genre is its acknowledgement that "as a young person, you can be respected for your ideas". In real life, she says, "young people are the ones who have to take on the world's problems and we're the ones coming into the world. You better listen. Because we probably have an insight that you don't have".

Growing up, Gregory thought she might be a police dog handler. At intermediate school, she missed out on a major role in Bugsy Malone because her twin brother scored the title and the love interest couldn't be his sister. At high school, in the finals of the Sheilah Winn Shakespeare competition, she didn't make the cut for the trip to the Globe Theatre in London, but she did visit Toi Whakaari: New Zealand Drama School.

"I walked into that building and I was like 'whoa - this is where I want to study' ... three years of drama school solidified that I wanted to be an actor."

Now Sydney-based, Gregory appeared in the first season of The Brokenwood Mysteries (her character comes back in season five) and was the lead in the docudrama The Monster of Mangatiti. Her work in that led to an audition for The Cul de Sac, where she competed with a flatmate from drama school for the role of Rose.

"She doesn't take any shit," says Gregory. "Aspirationally I would want to be more like her ... she's very empathetic but no bullshit is going to stand in her way."

In dystopian YA literature, women rule but often only at the behest of male characters - a stereotype The Cul de Sac avoids. This girl who is saving the world has no time for a boyfriend.

"It's funny," says Gregory, "People will accept a female hero in a dystopian sci-fi more than they would maybe accept it in real life?"

Stephen Campbell, The Cul de Sac's writer and director, says heroes are the hardest characters to create.

"With a male character, you can end up going down that cliche path of a bumbling anti-hero. You don't seem to do that with female heroes - you avoid the cliche." That, he decides, is a terrible answer.

Maybe women are the good guys in his science fiction dystopia because it's fashionable, "or maybe it's my upbringing?"

Campbell grew up in a blended family with four younger sisters.

"It was quite matriarchal. I don't know how much the men ... whether we had much say, really!"

The idea of a family that works to stay together with respect and humour is, he says, a legacy of that experience - along with Star Wars et al.

"I'm trying not to sound cliched ... as a young, hopeful film and television maker, people like [Steven] Spielberg and [George] Lucas always really motivated and inspired me. I always felt that kidult, family genre was the place to be."

He has a strong track record in making programmes for young New Zealanders. Campbell produced the first year of 3.45 Live (Thingee was an early product of his imagination), worked on Ice TV and more recently made Secret Agent Men and The Amazing Extraordinary Friends.

Young adult drama appeals because "the audience is much more open. Hyper-critical on one hand, but also once you have their trust and interest you can pretty much take them anywhere and the areas you can explore are wider. You can do fantasy and science fiction."

And, way back when, you could also start thinking about the oppressive nature of the cul-de-sac, when you bought your first house in Browns Bay. The drama's title came from that time, but its success is, perhaps, a sign of these times. According to some commentators the YA fascination with dystopia emerged from the 9/11 terror attacks - fiction helped adolescents face their fear the world might really end.

Campbell, 59, says it was a coincidence his villain was called Doni ("Trump wasn't even an idea in people's minds!") but as he started writing the third season he was thinking about international politics and the issues contemporary teens deal with.

"I grew up in a world where I was convinced we were all going to die in a nuclear holocaust. As a teenager, I didn't overly enjoy post-apocalyptic literature. I tended to look for more optimistic futures. When Star Trek came out, suddenly there was this future where we hadn't all died. We had gone on to become quite a decent species, and that was kind of interesting."

The Cul de Sac explores how far people will go to survive - in season two, after two of its stars (KJ Apa and Beulah Kole) scored Hollywood roles, Campbell had to write out their characters.

"What appeals to me is the idea of normal people, not superheroes, normal everyday people, suddenly finding themselves in a situation where they're having to deal with the big question of what life is about. Quite basic issues of survival, and how that tests your moral code ... we have characters who have sacrificed people to survive."

Campbell says "I'm old, but I do remember what it was like to be a teenager. All those worries, all that stuff you go through, that's all the same, but I think there's definitely more pressure today.

"Without being evangelical about it, our show talks about the importance of family, loyalty, sticking to your guns no matter how crazy the world is around you. If you hold onto those ideas, then you'll get through. You'll get through together. That's kind of our message, which I think is ... important. Hopefully, it helps."

The Cul de Sac's own survival is courtesy of New Zealand on Air. The Greenstone TV production picked up $1.2m of funding for its third season (comparisons are difficult, but season four of the adult drama Westside scored $6.5m; comedic entertainment show Jono and Ben picked up $1.7m). Campbell doesn't know if The Cul de Sac will have a fourth season but viewers can, he says, expect some closure for Gregory's character and family from the current storyline.

How long could a 25-year-old lead inhabit a teenage role? The sci-fi genre (and the show's writers) allow for shifting time and worlds, but in real-life suburbia, the Gregory who still remembers high school also acknowledges today's teens have it tougher.

"You know," she observes, "I was reading a tweet the other day about the rise of mental illness for youth, and how we'll most likely never be able to own a house, or property ... it's students who are dying in schools. It is a depressing world. It's kind of unbelievable really, what's happening around the world.

"I was looking at a 17-year-old friend's Instagram and was amazed at what is on there, I think it is hard. But I also feel like there is a movement of young people really trying to speak their minds ... Look at the Parkland students, the [school shooting] gun reforms, they are using Twitter in a way that I have never been able to think about, the way they are commenting on senators and stuff, it's just mind-blowing. Each generation probably thinks they've done their bit, but it's just huge what they are doing."

Seven signs

Need convincing The Cul de Sac is is a Kiwi dystopia?

• They eat pumpkin and it's not in a pie.

• The baked beans are Wattie's, the muesli Pams and Annie Whittle is playing the grandma.

• The "medic" learned everything she knows from the granddad she used to help out in the holidays. Doctor? "No, he was a dairy farmer."

• That's a public notice for the Puhoi to Warkworth leg of the Ara Tuhono state highway project on the back of the workshop door.

• The getaway van is ex-post office red and comes with an "I Love NZ" keyring.

• Guy falls over: "Just leave me ..." Female lead: "Don't be a dick ..."

• One of the safe houses is a shearing shed - and the obligatory sci-fi portal-to-another-place is made from an old trampoline.

Lowdown

The Cul de Sac, TVNZ 2, July 21