Australian Blake Johnston on Friday shredded the world record for the longest surfing session, admitting he felt "pretty cooked" after catching waves for more than 30 exhausting hours.

The 40-year-old former surfing pro broke down in tears after beating South African Josh Enslin's previous record of 30 hours 11 minutes.

In front of hundreds of cheering supporters at Sydney's Cronulla Beach, Johnston braved swarms of jellyfish and pitch-black seas that are home to many species of shark.

Johnston briefly thanked the crowd lining the beach during one of the short food and water breaks he was allowed, before paddling back out to try and push the record to 40 hours.

"I've still got a job to do. I said 40 so I'll go and give it a crack," he told reporters.

"I'm pretty cooked, yeah, but we'll push through."

Johnston's attempt started in the early hours of Thursday morning, using large spotlights to illuminate the water as he caught the first of more than 500 waves.

With Sydney in the grip of a minor heatwave, the water temperature has been hovering around a balmy 24 degrees Celsius.

Johnston went after the record to raise more than Aus$200,000 (US$133,000) for mental health, marking 10 years since losing his father to suicide.

He had originally planned to raise the money by tackling a 1,000-kilometre run, but settled on surfing when he saw the previous record was "only" 30 hours.

"I thought I could just do it," he said before the attempt.

"I push myself to the limits with my adventures and to prove to myself that I'm worthy and can get through hard times, and that's when my lessons are learnt."

He anticipated infected ears, dehydration and sleep deprivation would push his body to its limits.

Johnston's brother Ben said they had also prepared for the possibility of a shark attack, but it wasn't something that had worried them.

"I surfed at two in the morning with him and the lights actually went out so it was pitch black," he told national broadcaster ABC.

"There were a whole bunch of jellyfish out there, so it was interesting to say the least." (AFP)