Prime Minister Scott Morrison's claim that plastic pollution in the oceans is a more immediate threat than climate change has been trashed by Associate Professor Nerilie Abram, an Australian scientist who helmed the United Nations' report on rising sea levels.

In his statement to the United Nations early on Thursday (AEST), Mr Morrison promised more policies to clean up the ocean, deal with plastics recycling and crack down on illegal fishing.

"To protect our oceans, Australia is committed to leading urgent action to combat plastic pollution choking our oceans, tackle over-exploitation of our fisheries, prevent ocean habitat destruction and take action on climate change," he said.

Mr Morrison had already told reporters: "We need to take action on climate change, but there are actually issues like plastics in our oceans which present even more immediate threats.