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The 29-year-old Aucklander was having a training swim ahead of today's Classic Builders Tinman Triathlon when she became aware of something behind her in the water.
"I was on my way back to shore and stood up to see where I was and I looked behind me and saw something in the water. It took me about 10 seconds to realise what it was and then I began running into the shallows because the tide was out," O'Grady said.
"It chased me. The shark was coming behind me at speed and I made it to the beach. There was a guy on the sand who said he had been watching it follow me the whole way back up along the beach and he wished he had a camera so he could have filmed it."
O'Grady estimated from what he said that the shark had followed her for more than 100m.
She surprised herself just how relaxed she was and how quickly she reacted, considering it was her first experience with a shark.
"I first thought it was a swimmer's hand, then I realised it was a fin. It flaps like you wouldn't expect it would. It is not rigid like a dolphin's. The fin looked rather small but did not reflect the size of the shark.
"Someone had taken some selfies with the shark underwater, would you believe, about 10 minutes later and said it was quite a big shark.
"It did do the thing where it circled around. I am glad I did run to get back to shore and I was lucky it was quite tidal so could actually run in the water and didn't have to swim."
When she was safely back on shore the reality of what had happened - or could have happened - hit home to her.
"I was just shocked. I sat on the bank by my car just taking it all in afterwards."
O'Grady had a moment of panic at the start of the swim leg of the triathlon when someone swam past her quickly.
"All I saw was a black shape going past me and I did get a fright before I realised it was just a person."