Fossilised coral reefs in the Great Barrier Reef could help scientists understand how sea levels have changed over the past 20,000 years.

An international team of researchers will spend 45 days at sea, gathering core samples from about 40 sites.

Described as the "trees of the sea", coral have growth rings that show seasonal variations.

Researchers say the samples will also shed light on past sea temperatures, as well as other changes to the reef.