You'll barely believe what Austin did to save his family's lives.

It was meant to be a simple family day on the water.

On the Quindalup coast in Western Australia, Joanne Appelbee and her children had taken paddleboards and a kayak out into the ocean, enjoying what began as a calm afternoon.

But conditions quickly turned. Strong winds pushed them further and further from shore, until the land was no longer within easy reach.

That's when Joanne realised the family was in serious trouble.

"The wind picked up and it went from there," she told the BBC. "We lost oars, and we drifted out further.... It kind of all went wrong very, very quickly."

As she clung to her paddleboard with her two youngest children — Beau, 12, and Grace, 8 — she made a decision no parent ever wants to make. She turned to her eldest child, 13-year-old Austin, who was sitting on a plastic kayak, and asked him to leave them behind.

"One of the hardest decisions I've ever had to make was to say to Austin, 'try to get to shore and get some help' because it was getting serious really quickly," she said at a press conference, per The West Australian.

Austin was her only hope.

The family were lucky to be reunited. Image: 7News.

He paddled away from his mother and siblings and began the journey back to shore alone, battling powerful currents that had already dragged the family kilometres out to sea.

What followed was an ordeal that would test even the strongest adult swimmers.

The family didn't realise that the kayak was damaged and had already begun to take on water.

Austin lost an oar and started paddling with his arm. Eventually, the kayak gave out and the young boy slipped into the ocean and started swimming.

For the next two hours, he swam more than four kilometres through open water, using whatever technique his exhausted body would allow — breaststroke, freestyle, and survival backstroke — all while fighting fatigue and fear.

Halfway through, he abandoned his life jacket, which was getting in the way of his swimming.

Throughout the terrifying swim, he kept his mind positive, using prayer, Christian songs and "happy thoughts" to keep going.

Austin said he kept thinking of his family, repeating the same words in his head with every stroke: "Not today, not today, not today."

Marine rescue commander Paul Bresland later described what Austin did as a "superhuman effort".

When he finally reached the beach, his body gave out.

"I hit the bottom of the beach and I just collapsed," he said.

But even then, he wasn't finished. Austin got back to his feet and sprinted another two kilometres to find a phone, where he was able to call emergency services and raise the alarm.

By the time help reached Joanne and the younger children, nearly 10 hours had passed since the ordeal began.

They had drifted up to 14 kilometres offshore, alone in Western Australia's shark-infested waters.

All three were taken to Busselton Health Campus and have since recovered.

For Joanne, the wait was agonising. They were freezing cold, with no food and water, and the swell had begun to pick up.

As the hours stretched on with no word, she began to fear the worst — that her son had not made it back to shore.

"I had assumed Austin had made it a lot quicker than he had," she said. "As the day progressed, no vessels and nothing was coming to save us.

She kept thinking, "If he hasn't made it, what have I done? Have I made the wrong decision, and is anyone going to come and save my other two?"

The relief when rescuers finally found them was overwhelming.

Austin, who had passed out after his Herculean effort, was taken to hospital. When he came to, he had called his dad in distress, worried about the wellbeing of his mother and siblings.

Minutes later, he got the call that everyone was alive.

"It was a moment I will never forget," Austin said.

Joanne has since spoken about the importance of swimming skills, saying all of her children learnt to swim from a young age and urging other families to do the same. But when it comes to Austin, her pride is unmistakable.

"There's no bigger word to describe him other than proud," she said. "I'm speechless at his efforts, but at the same time, I knew he could do it."

Austin, meanwhile, has rejected the idea that he's a hero.

"I didn't think I was a hero," he said. "I just did what I did."

Feature image: ABC News.