The lower Yarra might not be every Melburnian's idea of a fishing spot, but try telling that to a hungry dolphin.

Drawn up the brown river by the prevalence of bream, four bottlenose dolphins spent the afternoon feasting and frolicking as surprised cyclists, rowers and office workers looked on.

Visiting from Port Phillip Bay, the four swapped saltwater for fresh and travelled as far up the river as Herring Island, between South Yarra and Richmond. They particularly enjoyed the area around Swan Street bridge.

Dolphin Research Institute executive director Jeff Weir said the mammals had been sighted as far up as Punt Road in the past, and had been seen in the Maribyrnong River recently.

But he said dolphins in metropolitan waterways was unique for such a large city. "Sydney Harbour lost their resident bottlenose dolphins years ago," he said. "What we've got here is special. There really aren't many places in the world where you've got 4 million people living around the waterway where these sorts of things still happen."