Giant sunfish spotted off Bondi Beach

One of the most mysterious creatures of the deep has made a rare appearance off the shores of Sydney's Bondi Beach.

A father and son were snorkelling and spear fishing when they encountered a 2.5m-long sunfish.

Aaron Puckeridge said he was drifting about 30m off North Bondi Point in 10m of water when his dad yelled out.

"(We) didn't actually know what it was to start with, because it was just this big object, motionless in the water," he told 9News.

"I looked right in its eye at one point, that was pretty much the only reaction I got from it, it actually kind of retracted its eye back a bit, and it disappeared behind all these folds of flesh."

Professor Culum Brown, head of the Fish Lab at Macquarie University, said it was very unusual for the deep ocean fish to come so close to shore.

"The only time they would really come into shallow waters, is specifically to be cleaned," Professor Brown told 9News.

"But to see it off Sydney, that's special.

"Most of these fish are huge, they're… a thousand kilos, so that's a one tonne fish, that's the biggest fish in the sea.

"The fact that it's coming into this nice clean water off the coast of Sydney, that bodes well for the quality of our marine life and marine eco systems in general."