Auckland swimmer Sarah O'Grady is vowing to never swim alone again after discovering a shark was pacing her warm up swim in Pilot Bay on Saturday.

She was swimming parallel to the beach before Sunday's Classic Builders Tinman Triathlon.

'A guy on the beach told me it was following me the whole way along the beach and we wish he had had a camera. So glad I didn't know about it until the very end of my swim,” says Sarah in a Facebook post.

'I didn't know it was following me it was only when I stood up at the end and it was there and then I ran in (tide was low) with it coming in hot behind me! Then a man on the beach told me it had followed me for ages!!

'I swam literally the loop we swam at Pilot Bay. Only found out from a spectator that the shark followed me the whole way back along the beach. I just saw the shark coming towards me after I stopped swimming and then it chased me into the shallows.

'This is why I don't go swimming alone. I was foolishly doing a pre-race swim by myself. Never again.”

The same shark starred in a video taken a few minutes later by another Tinman entry with Steven Morris from the Seamen Ocean Swim Squad who dived down to a fairly immobile looking shark on the harbour floor, and captioned it, 'Shark selfies in Pilot Bay with Seamen Phillips and Bruce The Bronze Whaler.”