Britain's George Nash and Will Satch took bronze as Eric Murray and Hamish Bond underlined their tag as favourites with gold in the men's pair.
The New Zealand duo, unbeaten in the last three years and lowered their own world record by six seconds in the heat, raced to victory in style.
France's Germain Chardin and Dorian Mortelette led through 500m before Murray and Bond swept past them.
Nash and Satch chased down the French but were beaten to the line.
"New Zealand have been outstanding - they have taken the pair by the scruff of the neck and won every race. Will and George were going over the buoys but what a fantastic performance and a brilliant bronze. That was one we didn't expect."
It was Britain's fourth rowing medal at London 2012 - later followed by
Katherine Grainger and Anna Watkins's gold in the women's double sculls
Alan Campbell's bronze in the men's single sculls.
"I can't really believe it," Satch said. "Six months ago I didn't think I'd even be coming as a spare. It is a dream come true."
A breathless Nash added: "It's unreal. Awesome."
The pair were only put in the pair boat earlier this year when coach Jurgen Grobler decided to move Andy Triggs Hodge and Pete Reed back into the men's four.
Hodge and Reed had failed to beat Murray and Bond in the pair boat over two years and Grobler chose to maximise Britain's gold medal chances in the four.
Nash and Satch won silver at the first World Cup of the year in Belgrade and their Olympic semi-final to show their potential of winning a medal.
The Britons, who trailed the French by a length after 500m, fought back to draw level in the closing stages, but Chardin and Mortelette responded to win the sprint finish.