Approached about a photograph showing the activity, Prof Liz Slooten, of the University of Otago zoology department, said a "veritable feeding frenzy" could be seen.
"The fish have almost certainly been brought to the surface by other, larger fish feasting on them from below.
"The seabirds are also getting into it from the top.
"I’m glad I’m not one of those fish seen boiling at the surface, trying to escape all these predators, attacking them from all angles," she wrote. The photograph was taken at 9.30am on Wednesday.
Dr Will Rayment, of the Otago marine science department, said humpbacks were fairly often seen off the Otago coast at this time of year.
The huge marine mammals were migrating north from summer feeding grounds in the Antarctic waters to their winter breeding grounds in the tropics.
They also fed along the way, by lunging through dense aggregations of krill or small fish with their mouths open.
Closing their jaws pushed the water out through their baleen, retaining the catch inside their cavernous mouths, he said.
Baleen is a filter-feeding system for non-toothed whales.
