Lydia Ward (14) struck the shark with her boogie board after it had bitten through her wetsuit and sunk its teeth into her hip.
She did not realise until she scrambled to shore that she had been bitten.
Her injuries were not serious.
"I think it's quite lucky that I did wear my wetsuit," Lydia said. "Because my brother was 2m away from me, and he wasn't wearing a wetsuit."
Shark expert Clinton Duffy, from the Department of Conservation, said the shark that attacked Lydia was most likely a broad-nosed sevengill, which had a "very good set of teeth" and was known to sometimes view humans as prey.
It could have been stalking her for a while before the attack, and was capable of inflicting "very serious injuries".
In Lydia's favour was that this shark would investigate its potential prey with its teeth first, and was easily dissuaded if a person fought back, Mr Duffy said.
Lydia was chest deep in water, waiting to catch a last good wave alongside 10-year-old brother Alex on Monday night when she stepped on something slippery on the sea floor.
"At first, I wasn't quite sure what it was. And I was trying to say to myself it was just a piece of driftwood," she said.
"Then I stood on it again, and I just looked at my brother's face and he said 'Whoa'. And I looked to my side and I just saw this massive grey thing twisting in the water."
The shark lunged at her and bit her.
Lydia thinks the adrenalin flowing through her body might have prevented her from feeling anything.
"So, I hit it with the end of my boogie board and that scared it away."
She could see blood in the water "but I couldn't run . . . so it was quite nerve-racking".
"I only realised it bit me when I hopped out of the water and saw the bite marks through my wetsuit."
Mr Duffy said Oreti beach had been the site of shark attacks in the past, but overall attacks were rare in New Zealand.
There are only 13 shark attack fatalities on record in New Zealand waters, and on average there are about two unprovoked shark bites a year, usually resulting in minor injuries.
Lydia and her brother estimated the shark was about 1.5m long, which Mr Duffy said was standard for a sevengill shark.
Lydia's mother, Fiona Ward, said her daughter was cool-headed by nature, and there was no screaming or hysterics.
Lydia said the attack had put her off going to the beach "but it's still fine with rivers and lakes. So, I'll just stick to them now".
