The huge mass was created by an undersea volcanic eruption near Tonga earlier this month.
A Tasmanian couple got a close-up view when it floated under their catamaran one night last week when they were near Fiji.
"It was dark so we were shining the flashlight on it and the whole ocean was matte. It was like gravel," said Larissa Brill.
Footage showed pumice to the horizon in every direction from their boat.
The 50km-wide raft contains millions of pieces, some as big as basketballs, and can easily be seen from space.
But while it's on course to collide with the Great Barrier Reef, it's actually a good thing for the endangered coral superstructure.
"When it gets here, [it will be] covered in a whole range of organisms of algae and barnacles and corals and crabs and snails and worms," Queensland University of Technology geologist Scott Bryan told ABC News.
"We're going to have millions of individual corals and lots of other organisms all coming in together with the potential of finding new homes along our coastline."
The raft will likely have broken up when it arrives sometime mid-next year.