Top jockey Michael Walker says he was struggling to walk three races before winning the New Zealand Derby aboard Puccini.

Walker injured his lower back while he was riding Rough Copy to victory in the Lindsay Mile, and was in some pain on his way back.

"Halfway up the straight something went `twang'. When I came back to the jockey's room I could not walk. I've never been in that much pain in my life," said Walker, whose suffered serious head injuries after a hunting accident in 2008.

At one point it looked like Walker could be stood down for the day. But with the aid of a massage and several painkillers, he made it to the line.

"I'm still in pain now," he said after the Derby.

Walker also had to extract himself from a position near the rear in a slowly run race on Puccini, who missed the start.

From there he let the horse move to the lead without working too hard, and he kicked on the turn to run away with the Derby by two lengths.

Walker has partnered many top gallopers in his career, including multiple Group One winner Darci Brahma, but he says Puccini is the best horse he's ridden.

"Horses don't do that, and I was still at full steam at the winning post. He could have gone another lap."