Ans Westra, pictured this year with some of her own work behind her.

Lisa van Hulst was 8 years old, waiting in the car outside a marae while her mother, Ans Westra, went on the grounds to see if she could photograph Māori gathered there. It was the late 1970s, when her daughter recalls: "Mum wasn't always welcome.

We would wait in the car while she went to feel the water, and sometimes be shouted at to leave. She wasn't invited or asked to photograph. It was something she did herself...

"There was a mixture of her being revered, and also questions about why she had come."

Every summer holiday during the late 1970s and early 1980s, Lisa and her younger brother, Jacob, were taken on their mother's photographic expeditions – to pā and marae around the East Coast; to Ruatōria, Ngāruawāhia and to Waitangi. The trio often slept on marae, or in houses they were invited into. On these trips, van Hulst recalls her photographer mother drove with a flask of coffee and mints to keep her awake. Her children slept – van Hulst on the back seat, while Jacob dozed on a pile of bags and the tent stacked up in the boot of their Valiant stationwagon.

Supplied Westra and daughter Lisa in Wellington, 1980.

Now 46, van Hulst sits in a Wellington cafe and describes her childhood as untraditional. Her parents split when she was 7. Home was a house in Karori, where Westra spent hours obsessively in her darkroom, but she also took her children on adventures. She never hung her own photographs on the walls.

The Dutch-born photographer (she came to New Zealand in her late teens) also has an older son, Eric, whose father was comic novelist Barry Crump. Her children learned about Māori culture the hard way. Says van Hulst: "We learned nothing about Māori in school. Mum threw us in the deep end, really. I remember staying in a marae one night and a kuia yelled at me because I was sitting on my pillow."

"We used to get hassled by the other kids on the marae. We were the Pākehā kids. Both our parents were Dutch, so we had funny accents."

Her mother was distinctive behind the camera, a tall, Dutch woman observing and capturing life from the edges. In what some have described as Westra's "Dutch obstinancy", van Hulst was often told off for getting in the way and ruining a shot. "She was always busy, really."

Ans Westra/Supplied Ngaruawahia, 1963.

Now a mother of two and the family trustee on the Ans Westra trust, van Hulst can appreciate the importance of the powerful black-and-white photographs of Māori her mother took over five decades. Westra is renowned for her long and productive career, one of the longest in the history of New Zealand photography. Considered part photographer and part social anthropologist, she took a staggering 220,000 images, of which almost half are held by the Alexander Turnbull Library, which has started to digitise the collection she bequeathed.

The cultural value of her work grows constantly. Between 1945 and 1986, the proportion of Māori living in New Zealand cities exploded from 26 to nearly 80 per cent. Westra captured this rapid urbanisation with her lens, providing the most comprehensive documentation of Māori culture over that 60-year period of significant change.

It wasn't just Māori she photographed, but Māori and Pākehā interacting in city streets for the first time.

In December, her first solo exhibition in the United States will open in a New York photography gallery. Alexander Turnbull Library also profiles her in its exhibition, Pūkana, a photographic study of Māori performance over the decades, while she is the only woman of eight photographers featured in the Te Papa publication, The New Photography. In October, a play about her life opened at the Nelson Arts Festival, with plans to tour the country.

Ans Westra/Supplied Ans Westra and her children Lisa and Jacob van Hulst at home in Karori, 1982.

All this comes at a time when the 83-year-old is in poor health. Westra's family and her dealer, David Alsop of Wellington's Suite Art Gallery, talk quietly about her health issues. Five years ago, she was diagnosed with dementia. She also suffers from bipolar disorder.

Art historian and documentary maker Luit Bieringa – who has made a film about Westra – says the dementia diagnosis has to be treated with caution. "We've visited her quite a lot since we were told that. We've found her sharp and sparky whenever we've seen her. She may have spells where she is in a depressive state. She has struggled with her own demons, and they have often affected her output and her relationships. We've come across that, but we've also appreciated her enormous output and energy when she is on top of it."

Van Hulst says: "She has good periods and not so good periods. She's pretty on to it in lots of ways. She would still love to do everything.

"She's been diagnosed with bipolar and you can see that pattern through her life. But she would deny it herself. Medication didn't really work for her. She'll say, 'I'm now in a low and I'll come out of it.' Then she'll come out of it, and say, 'That won't happen again.'

"Mum would have loved to go to New York for the opening but her health isn't up to it. It's such a shame, because if this opportunity had happened a decade ago, she would have definitely gone."

Ans Westra/Supplied Mt Eden Rd, 1971.

On a dismal morning last month, Westra slumped back in leather couch in the backroom of Alsop's gallery in Cuba St, dressed in a skirt and cardigan, black socks pulled up to her knees. She has a childlike quality. In some recent photographs, her fine grey hair is pulled into pigtails.

When it was time to go into the gallery for a video interview to promote the New York exhibition, she struggled to get out of the leather chair. Her back is hunched from years of bending over in the darkroom and looking down a camera. Her sister, Yvonne Westra, who came down from Whanganui to aid her with the interview, helped her out of the seat.

She lives in the Hutt Valley. She has friends and family who visit regularly, but is also known for keeping to herself.

On the day I visit, Westra is sharp and engaging. She still gets out with her camera, mainly photographing children in playgrounds and on the streets, and would like to collect these into another book. Her daughter says: "She definitely still takes photographs. I think that's the way she relates to people."

Ans Westra/Supplied Wellington, 1960.

Yvonne says her sister has been working on the childhood book for months. "She'll be photographing on her deathbed."

Many of Westra's iconic photographs of the 1960s and 1970s feature Māori children – playing or running outside, often without adults, usually looking happy and free. She says: "I like the way that children are so natural. They show their emotions openly. I like the way that children see the world."

Children featured in her controversial but best-known Washday at the Pā (1964) series – a bulletin produced for primary schools but eventually banned by the Education Department. The books were shredded, but the experience and the controversy surrounding it put Westra in the national frame. She spent just four hours with the Māori family of nine at their home in Ruatōria.

"I photographed everything they were doing. They were beautiful pictures but I was told we couldn't publish because it was not good for Māori. They were living in an old house with no electricity or water inside.

Ans Westra/Supplied Jerusalem 1963.

"To me, it showed the value of that way of life. The children used to help by cutting the firewood. It was such a loving family. I knew they were going to be moved to Gisborne, and their way of life would change, that they would start spending money for everything that they didn't need to do where they were.

"That sort of made my name at the time. Washday at the Pā became a political debate, the fact that Māori still lived like that, and they should be improving themselves and getting into special housing."

Bieringa talks about the significance of that series. "Māori culture was in decline. No argument. The fact that people like Ans highlighted the importance of Māori culture and rituals and traditions, making people aware that there were Māori in New Zealand. It was all part of the [preservation] process."

Throughout her career, Westra refrained from pointing her lens at the obvious targets. At the opening of the Waiwhetū meeting house in Lower Hutt in 1960, she snapped the crowd – Māori laughing and talking, Māori and Pākehā greeting one another, children playing, a girl reading a cartoon. She barely photographed Prime Minister Walter Nash and the Pākehā dignitaries.

Supplied Westra with her older son Erik, plus Lisa and Jacob, circa 1980.

Her photographs were published in Te Hou Hau, the Government-funded Māori magazine, and also the School Bulletin. "That gave me an introduction to photographing Māori. Nobody else was photographing Māori. People were not aware of their lifestyle, and what was going on at the marae."

"I wanted to be the fly on the wall who wouldn't be noticed. It was submerging into whatever was going on, observing it at the same time. Having my own ideas about what I wanted, but staying on the outside very much. I always wanted to tell stories about their lives."

She captured James K Baxter's funeral in the 1970s, Springbok tour demonstrations and a Porirua Mongrel Mob convention in the 1980s. However, from the late 1970s, her profile slipped. According to Bieringa, social documentary-style photography went out of favour until the mid 1990s.

Her daughter attended school in a secondhand uniform. "It was a pretty hand-to-mouth existence really during those years. Occasionally she would get a commission."

Ans Westra/Supplied Wairoa 1963.

On one commission, Westra photographed dams for the Ministry of Works's Think Big project of the early 1980s. Van Hulst travelled around the country, visiting dams. "I got to drive a 12 tonne roller," she says. "Those jobs allowed her to pay for what she really wanted to do. She was still photographing people."

The turning point came when Bieringa published a retrospective book, Handboek, in 2005, along with a documentary and touring exhibition. He felt Westra's significance could not be overlooked. Her years spent consistently at Ngāruawāhia, or at pā and marae on the East Coast were not always by invitation or welcome but they were undeniably brave.

"Ans took photographs without permission, but she was part of the scene. They're the only record of that period that really we have, which is astounding. They're unique images, and they're a real treasure. That legacy is terribly important."

In New York, at a gallery called Anastasia Photo, exhibition and projects manager Isabella Howard believes Westra's images are ones we can learn from today. The exhibition is called Urban Drift, the once popular term for Māori urbanisation from the late 1950s through to the early 1980s.

Ans Westra/Supplied Whakatāne, 1963.

Howard says: "Ans Westra's inimitable body of work struck a chord with the gallery for its longevity and context, as well as the realism and spontaneity in which Westra captures human interactions and experiences.

"In a cultural climate where migrant issues are at the forefront of the national discourse, the gallery felt showcasing another nation's experience would be well received by an American audience."

Of course, Māori reasons for heading to our cities were unique to New Zealand and remain far from resolved. But it happened. And Ans Westra was there with her camera.