There’s a reason graffiti art book Subway Art is referred to as “the bible”. By photographers Martha Cooper and Henry Chalfant, it’s an essential record of an ephemeral, urgent moment in street art, and for a generation of graffiti artists its images sparked a lifelong passion in the art form. Thirty-five years later, you’ll still often find a tattered copy on the bookshelves of street art fans worldwide.

Today, Cooper is sitting across from me in a quiet hotel bar in Melbourne. She’s a reserved, unpretentious woman in her seventies and she’s not a huge fan of the limelight. Words such as “legend” and “icon” make her squirm, and she’s uncomfortable signing the huge numbers of autographs asked of her. She tells me these days she has a tag, not because she fancies herself a street artist, but because she’s worried about people copying her real signature for credit card fraud.

Sitting next to Cooper is Selina Miles, herself a street art photographer. Miles’s feature documentary Martha: A Picture Story charts Cooper’s influence across the globe, from Sao Paolo, to New York, to Baltimore. It’s a fast and vibrant film, and a vital document of a legendary figure. When you look at classic New York graffiti art, you might not know it, but you’re probably looking at a Martha Cooper photograph.

In the 1970s, Cooper was a photographer at the New York Post taking “weather shots” – quirky street scenes, kids playing in the road, the kind of photos that fill a page on a slow news day. Then one kid she photographed opened his notebook, full of drawings, and introduced her to the world of street art.

His name was Edwin Serrano and went by He3. “He said, ‘Why don’t you take pictures of this?’” Cooper recalls. The boy was practicing writing his tag on a wall. Graffiti was everywhere by then, but Cooper didn’t know what she was looking at. “I could never read it,” she says. “But once I knew it was names – once I tuned in – I could see names everywhere.”

Serrano introduced Cooper to legendary American graffiti artist Dondi – given name Donald Joseph White – and the burgeoning world of street art opened up in front of her camera. Cooper was there at the beginning of a global art form, and she was one of only a handful of photographers taking it seriously at the time.

In the hotel bar, we look at one iconic photo she captured of Dondi straddling two subway cars, spray can in hand. It was dawn. They’d broken into a locked train yard and she’d photographed him painting a whole train, which took all night. It’s an energetic and irresistible image.

So why, all those years ago, did kids making illegal art in the Bronx want to talk to a white lady from the New York Post?

“Photography is a key part of it all,” she says. “They’d always been documenters of their own work, but they usually took them with these little cardboard cameras. And so I was able to give them pictures.”

Even in her seventies Cooper is far from finished. Her recent works include thousands of photos taken in her hometown of Baltimore. They’re bright and infectiously upbeat.

And she still lives dangerously, too. In the opening minutes of Miles’ film, Cooper is clad in all black, storming a Berlin U-Bahn station with Germany’s foremost graffiti crew 1Up. They’re armed with spray cans; she’s armed with a camera. They’re in and out in seconds.

“That’s the side that intrigues me the most,” Cooper tells me. “The idea that kids are willing to risk their lives for this, and they’re basically doing it for each other. It seems like a pure form of art to me, when they’re not worried about exhibitions or selling canvasses, they’re just out there writing their name with as much style as possible.”

Cooper turns to Miles, who is complicit too, having shot the 1Up scene. “Wouldn’t you agree the illegal stuff is more exciting?”

“There are real stakes!” says Miles. “Will it work? Will it not work?”

“I photograph artists painting murals, but you can stand in front of them all day long and only a little piece will get finished,” Cooper adds. “You’re literally watching paint dry.”

When she’s not travelling the world tailing renegade street artists and careening away in getaway cars, Cooper tells me she’s playing Pokemon Go. She seems vaguely embarrassed about it, but it sounds like a similar pastime to me: travelling to often odd and neglected corners to capture something colourful, strange and fleeting.

The art form has changed a lot over the years – it’s now widely accepted as art, and Cooper’s eyes light up as she tells me about a graffiti workshop in a museum in Sacramento recently. “Graffiti writers were showing adults, not just kids, how to paint with a spray can,” she says. “People in suits. Inside a museum!”

But the one thing Cooper doesn’t want it to lose is that sense of urgency.

“It’s fleeting,” says Cooper. “That’s the reason photography is important. In the end, it always lasts longer than the walls. The artists know it won’t last. If it’s illegal in the street, it might not last the night. Way more people will see them in photographs.”

The pieces in Subway Art are all gone, as are the huge tags 1Up left on the Heinrich-Heine-Straße U-Bahn platform. But Cooper’s photographs – and Miles’ film – will last forever.

Martha: A Picture Story is screening in cinemas nationally.

Watch the trailer.