Then she received a phone call that changed her life.

The photograph that she had captured on a grey, Melbourne day last year, of a woman contemplating her aged cat, had been chosen as the winner of the National Photographic Portrait Prize.

From around 1220 entries, which had been whittled down to the 53 entries that are now on display at the National Portrait Gallery, her subtle, intimate portrait had stood out to the judges as “a work that gives more to the viewer the more they look at it”.

The image shows Low's friend, the artist Yhonnie Scarce, with her ailing, 19-year-old cat Indiana.

Scarce had asked Low, a 22-year-old TAFE graduate based in Melbourne, to capture the portrait when Indiana's health deteriorated, and the cat died just a few days later.

“It's not staged at all. I have a visual diary and I generally tend to sketch everything out beforehand before I do a shoot, and I had an idea,” Low said.

“But that shoot revolved around Indiana. She was a diva when she was little and she was a diva right at the end. She only weighed about 2kg at the time, and when I patted her my hand just sank through her fur to her bones. But she was still dictating the shoot.”

She said the portrait was typical of her style.

“I'd like this to be a bit of a statement of what I do from now on,” she said.

“It's a very straight photograph. I like chasing natural light rather than setting up the shot, and you couldn't do it for something like this, a quiet, intimate scene.”

Artist and writer Martyn Jolly, who was this year's guest judge, said the work was beautifully composed.

“It references vanitas painting, Dutch interiors, all that is there, and then there's actually a particular personal story,” he said.

“But it's not heavy-handed. It's moving - age and youth together, even cross-species.”

Gallery director and judge Louise Doyle said it was a different take on the reality of ageing.

“You have a sense that the pussy cat is a mature being, and having a struggle in that moment, and it's such a tentative beautiful thing, the resolution of the owner - she's there knowing that at some point, the cat's not going to be there.”

Curator and judge Joanna Gilmour said the portrait was one of many in the show that worked on various levels.

“You've got the technical proficiency, and the appeal of the lighting and the palette and the composition, but then when you really learn about the work, there's so much of it that resonates.”

Just hours before she was to be presented with the $25,000 award from Prime Minister Julia Gillard on Friday, Low said she was still reeling.

“I was just thinking I might have to put photography aside for a little while, just to get by and just to live,” she said.

“But now I will just be able to live and to shoot and continue my work and developing and really extending my practice.”

The National Photographic Portrait Prize exhibition is on display at the National Portrait Gallery until May 19.