The 16-year-old first went snorkelling with his sister, coming within a metre or so of the sharks, and described the experience as "good fun".
When the wind died down, he used a drone to get a better picture of how many sharks had swarmed into the bay.
"The biggest one I saw [while snorkelling] was maybe four or five feet but when I was using the drone I could see some big ones," he said.
"There were so many different types of sharks."
Among them thresher sharks as well as the hammerheads.
"We finally found out why the fishing has been so bad," he said.
"Everyone who is out fishing is catching sharks," his mother Sarah added.
Locals had remarked that was all they were catching.
"We went out paddleboarding again with them this morning and it actually is quite unnerving when you are suddenly paddling over these sharks," she said.
"There were a pile of kids out there yesterday sea-biscuiting but they all changed their minds when they saw how many sharks there were."
For just over a decade, the family has been spending the summer at Great Barrier Island but they have only seen a school of sharks like this once before.
The 16-year-old has previously filmed the impressive phenomena but not on this scale.
Previously there might have been a hundred or so but this year there was "probably a couple of thousand," he said.
In 2018, Department of Conservation shark scientist Clinton Duffy told the Herald it was not uncommon to see schools of baby hammerheads in the area.
"The Hauraki Gulf and the Firth of Thames are one of the largest hammerhead nursery grounds in New Zealand that we know of," he said.
"The west coast of the North Island, the Hauraki Gulf and parts of the western and eastern Bay of Plenty are the hotspots for baby hammerheads."
Young hammerheads tend to like warm, shallow, productive waters.
