“I haven’t seen anything like this before,” said Layne Beachley, a former world champion Australian surfer, referring to the scale of the paddle-out protests.

“To see so many surfers standing up and using their voice and position to let the people in power know we’re vehemently against this proposal is really powerful,” Ms. Beachley said.

The Bight has been called Australia’s answer to the Galápagos: a stretch of ocean facing the Antarctic that is home to calving whales and teeming fisheries. The Bight’s marine environment is starting to be recognized by scientists as part of a “Great Southern Reef” that rivals the Great Barrier Reef for ecological value.

But experts say the Bight also holds one of the world’s great remaining natural gas reserves, and Equinor, a company controlled by the Norwegian government that was once known as Statoil, plans to begin drilling by the end of this year. Equinor changed its name to move away from the word “oil” as it stresses investments in renewable energy.

The plan to extract natural gas still requires approval by Australia’s offshore oil and gas drilling regulator, and Equinor insists that exploration will be safe.

In Australia, the fossil fuel industry has a long history of political connections. And candidates who support the proposal argue that opening the Bight to drilling would create jobs and wealth for decades to come in the struggling postindustrial area of South Australia.

But many people fear that the Bight, much of which extends along the state of South Australia, could face catastrophic damage in the event of an oil spill.