Eric Nerhus described feeling the shark's teeth dragging across his body.
His head, shoulders and one arm were inside the shark's mouth during the attack, off south-east Australia.
Mr Nerhus, 41, says he survived by feeling for the shark's eye socket and stabbing with his fingers, prompting the shark to let go. I've never felt fear like it 'til I was inside those jaws, with those teeth getting dragged across my body," the abalone diver told Australia's Channel Nine network.
He spoke from his hospital bed a day after Tuesday's attack, which took place off Cape Howe, some 400km (249 miles) south of Sydney.
Mr Nerhus said he was collecting abalone when the shark attacked him, dislodging his oxygen supply from his mouth before partially swallowing him.
"Half my body was in its mouth," he said.
"I felt down to the eye socket with my stiff fingers. I poked my fingers into the eye socket, which the shark reacted to in a way that it opened its mouth a bit, and I just tried to wriggle out."
He described how he felt "the teeth crunching up and down on my weight vest."
Experts have said that there is a possibility the shark mistook the wetsuit-clad Mr Nerhus for a seal.
"Normally they feed on seal [...] so it's bitten in on this guy thinking he's a seal," shark specialist Grant Willis said.
He said that when the shark realised Mr Nerhus was not a seal he may have spat him back out again.
But Mr Nerhus said he was glad his survival instincts had kicked in.
"I couldn't think of a worse way to go than to end up as fish food. That's why I fought back. I was determined I didn't want to go like that. I like life too much."
Shark attacks are not uncommon in Australian waters, the BBC's Nick Bryant in Sydney says.
There are around 15 a year - one of the highest rates in the world. An average of one a year is fatal.