Size matters for the world's most threatened seabirds, who are at risk of death from even the smallest pieces of plastic floating in the world's oceans, a study has found.

Researchers measuring the size of plastic rubbish eaten by the petrel family of birds in Australia and New Zealand found 90 per cent of the plastic trash trapped in the bird's gut was just two millimetres to 10mm in size.

Ninety per cent of the plastic trash trapped in the typical petrel's gut was just two to 10mm in size, much smaller than originally thought. Fragments of plastic less than 10mm are the most abundant floating litter in the ocean. Credit:File photo

"These sizes are much smaller than we originally thought," University of the Sunshine Coast marine biologist Kathy Townsend told AAP.

"Worryingly, fragments of plastic less than 10mm are the most abundant floating litter in the ocean."