"There were quite a few people swimming around the bay, only a few metres away from some of the sharks," he said.
"But they weren't really intimidated by anyone. They were just swimming around freely."
He said the sharks were about 3-4ft in size, and stayed at the bay for about a day.
Despite holidaying at Great Barrier every Christmas, Zach said he had never seen that amount of sharks in the bay before.
Department of Conservation shark scientist Clinton Duffy said 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"
He said the sharks featured in the video were juveniles and posed no risk to swimmers.
"Looking at the size of them they are not newborn, they are probably at least a couple of years old, but the further you get into the Hauraki and up into the harbours, the smaller they will get," he said.
"They move around in fairly loose schools, and you can get large concentrations of them in particular areas at particular times.
"They tend to like warm, shallow, productive waters."
Duffy said not much was known about the smooth hammerhead shark, but they fed on small flat fish and schooling fish, squid and octopus.
