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The incident occurred about 8.30pm yesterday, as hundreds of people enjoyed an evening on the beach.
Oreti Surf Life Saving lifeguard Timothy Ball said he and other club members were training when they were approached to help the young woman.
The off-duty lifeguards arrived to find her bleeding from a circular wound on her upper right thigh.
Friends had already applied a tourniquet.
She was conscious and in good spirits despite ''waves'' of pain, Mr Ball said.
She told them she had been boogie-boarding when she felt a pain in her leg,
The cause of the wound had not been confirmed, but, despite some media reports last night, Mr Ball said it was not a shark bite.
It appeared similar to the stringray wound inflicted on Jamie Cunningham (48) last month, also at Oreti Beach, Mr Ball said.
Mr Cunningham had also been body-boarding when he stood on a stingray while leaving the water.
Last night's victim was taken to hospital in Invercargill with minor injuries.