This beachside snap of a young couple at an engagement party ended up saving the life of a total stranger who was drowning in the waves behind them.

Jackson Parr posed for the snap with girlfriend Madison McDonald as the pair celebrated his cousin Harrison Day's engagement on Saturday.

But it wasn't until he glanced down at the picture that he heard chilling screams coming from beyond the shore at Austinmer Beach, 70km south of Sydney, Australia.

Parr sprinted towards the surf and past a woman yelling that her partner was drowning out at sea.

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Diving into the waves, Parr then swam for five minutes before reaching David Pelman, who had been caught in a rip.

Recalling the terrifying moment on Facebook, Parr posted: "Every 20 seconds or so I could hear this muffled 'help' so I just kept swimming trying to swim towards the voice."

"I held him up treading water for about five minutes until another guy found us out there and we grabbed him by his hair and started swimming for shore."

Two other guests at the party, Shaun Blair and Ryan Prain, made their way to the troubled pair on surfboards and aided them back towards the shore.

Parr said: "We had drifted out so far away from the beach but the rocks were a lot closer so we said let's swim for the rocks as getting smashed up against them might be our best option and that's what we did.

"There was a group of guys from the engagement party waiting on the rocks and helped us after the waved picked us up and dumped us on them."

A fourth rescuer, Scott Kohlhardt, leapt into the water as the three men struggled to keep Pelman's head above water.

/// Do not swim at night /// Last night I was at my little cousin Harrison Day engagement party and we went outside to take this photo 📷 and as we got the phone back and looked at the photo I heard this scream in the distance which made me look up. And what I saw was a woman hysterically screaming running up from the surf and instincts just kicked in. Screaming woman. 9pm. Beach. Someone must be drowning. So I started sprinting towards the surf and as I ran past the woman she screamed my partner is drowning. I ran into the surf and just started swimming as fast as I could 🏊‍♂️ every 20 seconds or so I could hear this muffled “help” so I just kept swimming trying to swim towards the voice. After about 5 minutes I found the guy who was floating on his back Choking on water. And he couldn’t swim and was stuck in the rip. I held him up treading water for about 5 minutes until another guy found us out there and we grabbed him by his hair and started swimming for shore. We had drifted out so far away from the beach but the rocks were a lot closer so we said let’s swim for the rocks as getting smashed up against them might be our best option and that’s what we did. There was a group of guys from the engagement party waiting on the rocks and helped us after the waved picked us up and dumped us on them. If you can swim, if you can’t swim, please please please don’t go swimming at night. There were a million different ways that story could have had a horrible ending. I’m so grateful I was in the right place at the right time to help 🙏

A post shared by Jackson Parr (@jacksonparr) on

Kohlhardt told The Leader: "I jumped in with a board. The guy was so big, I was worried he was going to grab Ryan and pull him under.

"He was fighting for his life. We were just telling him, 'Mate, you've got to try to hold onto the board.'"

Eventually everyone was pulled to safety, with a team of paramedics having arrived in time to treat Pelman.

Kohlhardt added: "I think we all shook hands, and we handed him over to the paramedics, and then we went back to the party because they wanted to get the speeches under way."

Parr warned people not to go swimming at night.

He wrote: "If you can swim, if you can't swim, please please please don't go swimming at night."

"There were a million different ways that story could have had a horrible ending. I'm so grateful I was in the right place at the right time to help."

Pelman also thanked his rescuers in a social media post on Sunday.

He wrote: "Thanks heaps bro wouldn't be alive if it wasn't for you and the boys you were with GOD BLESS YOU."