The photograph, taken by German artist Andreas Gursky in 1999, is now the world's most expensive after an auction at Christie's New York last week.

The record price for the 1.8- by 3.6-metre image exceeded pre-sale estimates of $US2.5 to $US3.5 million, Christie's said.

American artist Cindy Sherman previously held the record for the most expensive photograph, Untitled #96, a 1981 self-portrait that sold at a Christie's auction for $US3.9 million in May.

Peter Lik holds the record for the most expensive photograph by an Australian, when his image of the Androscoggin River in New Hampshire in the United States, One, sold for $US1 million in January.

Gursky was already popular with art collectors before Rhein II. His 99 Cent II Diptychon photograph of supermarkets sold for $US3.35 million in 2007.

The artist's multimillion-dollar images were examples of how investment in photography, like art, has continued to remain buoyant despite global economic jitters.

"Despite the debt crisis engulfing Europe and an Occupy Wall Street protest directed at Sotheby's, life inside the art bubble remained effervescent, buoyed by a marathon of flawlessly orchestrated parties, invitation-only dinners and blue-chip openings," The New York Times noted last week.

"In some cases it's a bit of a herd mentality," said Australian photography critic Robert McFarlane about why the price tag on Gursky photographs had increased.

"Gursky's attraction is probably in scale and detail and the fact that the market looks for successes to what sold before," McFarlane said, citing the 99 Cent II Diptychon image.

"And if they know that Gurksy sold for [$US3.35 million], they have a fair idea that a new or slightly different work may challenge that price.

"I don't think there are many of [such expensive photographs]. They are fairly rare. How they actually reach that point is a gradual process ... But once it becomes part of the 'accepted excellence', the price can escalate quite quickly."

The winning bidder for Rhein II was not revealed.

Other Gursky photographs are featured in the Museum of Modern Art in New York, the Centre Pompidou in Paris and Tate Modern in London.

The photograph was previously owned by a "distinguished private German collector", Christie's said, although it did not add how much he or she had bought it for.

And while some people may question why anyone would pay millions for such a stark, seemingly plain image, McFarlane said Gursky's work was respected for its "reverse exploration of the poetic" and its take on today's super-consumerist society.

"It's very simple, very graphic. In a funny way it makes you contemplate the very ordinary moments in your life," McFarlane said about the 1999 photograph.

"It's a path, it's a water course and it's the sky. And each of these things are archetypes in the way that we see life. Great artists can take the very ordinary and make them extraordinary."

Gursky has called Rhein II - one of an edition of six photographs - his favourite image, saying it was an "allegorical picture about the meaning of life and how things are", The Guardian reported, quoting a previous interview.

The artist removed all features in the photograph, including a building, dog walkers and cyclists, the newspaper said.

Culture journalist Florence Waters wrote in London's Daily Telegraph that Gursky's photograph represented the growth in the late 1980s of the photograph as art.

"The scale, attention to colour and form of his photography can be read as a deliberate challenge to painting's status as a higher art form," she said.

"On top of that, Gursky's images are extraordinary technical accomplishments, which take months to set up in advance, and require a lot of digital doctoring to get just right."

For McFarlane, Gursky's work represents the austerity of 21st-century German art. The artist took the Rhine, the lifeblood of German industry, and portrayed it in the most minimalist way possible, he said.

"This picture in its simplicity is really quite contemplative and oddly calming. In an odd sort of way, you could live with this picture.

"But I personally hunger for more lyricism ... I think [Gursky's works] are a completely valid statement. But they don't touch the heart. They touch the head. The heart gets left behind."