Warning whistles have evacuated a beach in Forster, with swimmers not being warned of sharks, but of a swimming brown snake.

Swimmers at Great Lakes yesterday were forced to exit the water as a brown snake was seen leaving the water, right between the flags.

Visitor Olivia Moffatt told the Great Lakes Advocate that beachgoers immediately abandoned their summer activities when lifeguards sounded a warning.

"Tourists and locals fled from the water after the whistles were blown. At first people were concerned it may have been a shark, only to discover a 1.5m brown snake," she said.

Swimmers fled the water as lifeguards blew their whistles. (Great Lakes Advocate)

Facebook users were quick to voice their own warnings when Great Lakes Advocate shared a picture of the snake on its Facebook page .

"There was a brown snake sunning itself a few weeks ago on the new footpath outside of the One Mile Surf Club, but the very brave surf lifesaver gave it a squirt from the hose and it slithered back to the dune,” said one reader.

"I have seen heaps of them over the years at Seal Rocks and Mungo Beach when fishing,” said another.

Ms Moffatt and other swimmers on the busy beach thought it was just a sea snake.

The bite of a brown snake can be fatal if not treated quickly enough.

Local snake expert John Smith assisted by telephone, and advised lifeguards to remain behind the snake.

"(The lifeguard) said (the snake) was moving south towards the rocks and that he would call back if he needed anymore help," Mr Smith said.