"I managed to shout to Dean as I was being pulled deeper into the water, shaken from side to side. My body went into such severe shock that as the shark bit me I didn't even feel pain."
She was only saved by the bravery of her husband, 40, a company director, who risked his own life to punch the shark on the nose three times.
The couple, now back in Ealing, west London, with their children Katie, 11, and Louis, seven, were at idyllic English Bay, just 15 yards from the shore when the shark struck.
Mr Gonsalves said: "I heard a petrifying scream. I looked up and spotted the shark with Frankie's leg between its jaws.
"It all happened in a terrifying but calm slow motion. I dived under the water, then punched the shark between the eyes. The shark seemed stunned because it let go of Frankie's leg. I then shouted to Frankie, 'Swim!'
"But it had bitten through her tendons and her foot was almost severed at the ankle."
She said: "I couldn't swim properly as my foot felt as if it were dragging. I had no idea then how bad the injury was.
"I was shouting for help to bystanders. I was petrified Dean would also be bitten by the shark, and I could see the children crying as I desperately tried to make for the shore."
Meanwhile, the shark had turned round and was swimming back to her husband.
He said: "Every fibre in my body told me to turn round and simply swim for my life.
"But I instinctively knew that would be disastrous as I could never out-swim a shark.
"So I turned round, treading water so I could see where the shark was. Within minutes the shark was swimming towards me and I punched it again on its head. I was shocked at how hard it felt. It was like hitting a metal bar with sandpaper over it."
Once again the shark fled. "But it came back a third and fourth time," Mr Gonsalves said.
"Each time I punched its head and it swam off. After what seemed a long time, but was probably minutes, I felt bystanders drag me out of the water, so exhausted I could barely stand."
At the local hospital, doctors tried to save his wife's foot. "When I saw my foot hanging off I was shocked," she said.
Five days later she was flown to Britain where she had more surgery at St Mary's Hospital in London. She is expected to recover within six months.
Mrs Gonsalves says the family, who were travelling back to Britain when their ship docked at Ascension, intends to return to St Helena.
She said: "We feel incredibly lucky to be alive. Dean is not only my husband, but my hero."