17-year-old Oscar Hugill and his friend Liam Doughty, 16, were relaxing on a sandbank in the ocean at the south end of Narrabeen beach in Sydney's northern suburbs when they noticed a commotion on the sand.

"All of a sudden we looked into the shore and there were all of these people signalling to a boy and his dad, we thought it was a shark attack," Oscar told Northern Beaches Review.

"We started to run over to see what was going on, when we got within 40 metres from them we realised they were drowning... or on the verge of."

About 50 metres from where Liam and Oscar had been sitting, a 6-year-old boy and his dad were struggling to stay afloat in the water. The boy had got caught in a rip and his dad went in after him to help. He was able to hold his son up, but he couldn't get out of the rip.

"I saw Oscar run along the sandbank and I followed behind him, and Oscar being a little bit smaller than me, I told him to get the kid and I would help the dad," said Liam.

"I was really concerned about the kid because he was not in great shape at all, but I reassured the dad it would all be okay."

Another bystander came over with a surfboard and helped Liam take the dad into shore and Oscar carried the son in. The whole ordeal was over in a matter of minutes.

"It was hectic, it was a massive adrenaline rush," said Liam.

"Once we brought them into shore we realised how badly that could have gone, everyone was saying there were no experienced swimmers around."

Julie Kimberly, a witness to the event, shared the boys' story on Facebook where it quickly went viral. She said Oscar and Liam "didn't think twice" about coming to the aid of the struggling swimmers.

"We all truly thought we were going to watch two people drown," she said.

Oscar says being recognised for what they did is "pretty cool" and Liam said they were "trying to stay humble."

"If anyone else was in our position I am sure they would do the exact same thing, it was just lucky Oscar and I were both experienced swimmers," said Liam.

Only three beaches in the northern beaches are patrolled year-round, Narrabeen is not one of them. Oscar and Liam say surf education in school and through nippers gave them the confidence and knowledge to help out.

"A bit of beach knowledge is definitely essential. Knowledge of the beach and currents and rips, and how quick they can really get you," said Oscar.

"It's good to be there for other people because I know I've been in that situation, where I've been out of depth, I know how scary it would be and it's good being able to help someone else."