A couple who were yachting in the Hauraki Gulf found themselves surrounded by fins in the water as far as the eye could see.

Alison Meredith and her husband sighted a large school of sharks, believed to contain hammerheads, off Army Bay in Auckland about midday.

They were nearing the end of a journey from Mahurangi Harbour to Army Bay and were moving slowly, about 2 knots, when they realised they were surrounded by sharks.

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"We looked out of the hatch and saw a whole lot of fins."

For about 20 minutes these could be seen in every direction.

"As far as the eye could see sort of thing," Meredith said. "It was exciting."

There were only a couple of spots where they could see through the water clearly enough to confirm that one was definitely a hammerhead.

"Although the fins all looked similar. They weren't very big."

She believed there could have been hundreds in the school.

Department of Conservation shark scientist Clinton Duffy told the Herald it was common to see schools of baby smooth hammerheads in Hauraki Gulf.

"It is one of the more important nursery areas in New Zealand for hammerhead sharks.

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"Hammerheads commonly school together, especially the juveniles."

Adult females measuring more than 3m moved into the gulf to pup, then likely moved into the outer gulf where they feed mainly on schooling fish, squid and octopus.

Duffy said it was unlikely to be the same school that was sighted off the Great Barrier Island as that group appeared to be mostly comprised of school sharks.