"It's exciting stuff. It was one of those real 'eureka!' moments for us," said Otago zoologist Associate Prof Jon Waters, who led the Marsden-funded study.
The Otago study was published this week in the respected British journal Proceedings of the Royal Society series B.
Researchers involved in the study used genetic evidence to ascertain, for the first time, the origins of bull-kelp plants washed up at St Clair beach.
They discovered that several of the kelp specimens, some found early last year, and others in May this year, had travelled from the subantarctic islands - from as far away as the Auckland Islands, 600km south of mainland New Zealand.
Other seaweed was likely to have come from the Snares Islands, 390km south of the mainland.
Kelp anchors itself to the sea floor with a hollow, root-like structure called a holdfast, which is also home to many marine organisms, including worms, sponges and crabs.
When the kelp breaks off and floats away, the organisms go with it.
Prof Waters said a wide variety of rocky-shore animal species were shown to raft with the kelp by clinging on, or by living within burrows and crevices in the "holdfast".
The St Clair beach finds involved the longest proven distance of ocean travel for animals hitchhiking on such naturally-occurring rafts.
Dr Raisa Nikula, a postdoctoral fellow also involved in the research, said this was also the first study to demonstrate that a whole group of marine animals was travelling together.
Within some of the holdfasts, researchers found an ecosystem which included 10 species of marine invertebrates, including two tiny crustaceans, a sea spider, several species of molluscs and a sea star.
Another postdoctoral researcher, Dr Ceridwen Fraser, who co-authored the study, said the kelp's strong buoyancy and toughness enabled it to withstand long periods adrift in the ocean.
Bull-kelp plants that have been drifting in the ocean for a long time often have large stalked goose barnacles growing on them. The size of these barnacles indicates the time the kelp has been at sea.