A 13-year-old boy swam for nearly four hours through cold, rough waters to save his mother and two younger siblings after they were swept out to sea off the coast of Western Australia, police said.

The family, from Perth, were kayaking and paddleboarding on Friday morning when strong winds and choppy ocean conditions began dragging them farther offshore.

The teenager, Austin Appelbee, swam about four kilometres to shore to raise the alarm.

“The waves are massive and I had no life jacket on. I just kept thinking... just keep swimming, just keep swimming", Austin said.

“And then I finally I made it to shore and I hit the bottom of the beach and I just collapsed.”

Austin said he initially set off in an inflatable kayak while wearing a life jacket, but abandoned both after the kayak began taking on water and the jacket slowed his swimming. 

He said he focused on staying positive as he made his way to shore.

The family had entered the water before noon. Austin raised the alarm at around 6 pm, and a police helicopter located his mother, Joanne Appelbee, 47, his 12-year-old brother Beau and eight-year-old sister Grace at about 8:30 pm.

Authorities said the family had drifted around 14 kilometres from Quindalup and spent approximately 10 hours in the water.

“The actions of the 13-year-old boy cannot be praised highly enough, his determination and courage ultimately saved the lives of his mother and siblings,” Police Inspector James Bradley said.

Joanne Appelbee told reporters she made the decision to send Austin for help because she could not leave all her children stranded at sea.

“One of the hardest decisions I ever had to make was to say to Austin, ‘Try and get to shore and get some help. This could get really serious really quickly,’” she told Australian Broadcasting Corp.

She said she was initially confident he would reach land, but grew increasingly worried as night fell and no rescue arrived.

“We kept positive, we were singing and we were joking and we were treating it as a bit of a game until the sun started to go down and that’s when it was getting very choppy. Very big waves,” she said.

By the time they were rescued, all three were shivering, and Beau had lost sensation in his legs due to the cold, she added.

“I have three babies. All three made it. That was all that mattered,” she said.