An Australian art teacher's photograph of a young Aborigine who was once his student won the country's top photo portrait prize, with judges praising the black-and-white image's intimacy and power.

Scott Bycroft's Zareth won the A$25,000 (€16,373.77) 2010 National Photographic Portrait Prize from the National Portrait Gallery in Canberra, beating 43 finalists selected from 1,000 entries submitted by amateur and professional photographers.

The image shows Clontarf Aboriginal College teenager Zareth Long leaning on a post, bare-chested and his wavy hair framing his face, gazing into the camera.

Mr Bycroft told local media that he had taken the shot at a swimming carnival at the school in Western Australia and that Mr Long had since started work at a mining company and was the father of a new baby.

"It was all spontaneous," Mr Bycroft told WA Today. "I always have my camera with me at school and Zareth was watching an event. He was already in that pose. He didn't change it and just gazed into the camera on my approach."