A teenager had to hold his mate's head above the water as he struggled to make it back to land after an extremely close encounter with a large shark off a Dunedin beach last night.

St Clair Surf Life Saving Club members Aidan MacRae, 13, and Chris MacDonald, 15, were swimming back to St Clair beach after training when they were brushed by what they say was a shark at least 2m long.

"We were slowing down and I kept looking back to check on [Aidan] and that's when I felt something rub on the inside of my right leg," Chris said.

Aidan said once he saw the shark he panicked and could not communicate with Chris. "I was trying to tell him to go back to Second [Beach] but he couldn't understand what I was saying." Once over the initial shock, Chris started to yell for help.

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Someone on shore heard his calls and called the police — that was when the boys tried to swim back in.

But Aidan was unable to hold his head above the water any longer, so Chris had to hold him up as they swam to some nearby rocks.

"I pretty much stripped off his wetsuit because he seemed hypothermic and we waited for people to come and get us," Chris said.

Despite the close call the pair will be back training in the water today.