Researchers have been forced to avoid crocodiles, deadly jellyfish and even quicksand-like mud to replant seagrass beds south of Cairns as part of a project they hope to expand across tropical Australia.

The underwater meadows at Mourilyan Harbour were once thriving habitats, providing food for prawns, dugongs and green sea turtles.

But the three hectare-site was destroyed more than a decade ago by a series of La Niña events, including Cyclone Larry, and the sand banks it needed to regrow washed away.