Austin Appelbee poses for a photo in Gidgegannup, Australia, on Tuesday, after the 13-year-old made an hourslong swim to raise an alarm after his family was swept out to sea off the Australian coast. (Briana Shepherd/ABC/The Associated Press)

A 13-year-old boy is credited with saving the lives of his mother and two younger siblings with an hourslong swim after the family was swept out to sea off the Australian coast.

Austin Appelbee swam four kilometres to shore to raise the alarm after he got into difficulties on Friday with his mother, Joanne Appelbee, 47, brother Beau, 12, and sister Grace, 8, police said.

Austin said he initially set off for help on an inflatable kayak that was taking water. He abandoned the kayak, then took off his life jacket because it impeded his swimming.

He said he tried to focus on positive thoughts as he swam for around four hours through rough seas for shore, raising the alarm at 6 p.m.

“The waves are massive and I have no life jacket on. … I just kept thinking, just keep swimming, just keep swimming,” Austin said on Tuesday. “And then I finally I made it to shore and I hit the bottom of the beach and I just collapsed.”

This image taken from video shows Austin Appelbee, right, posing with his family on Tuesday. (ABC/The Associated Press)

The family, from the state capital, Perth, were on vacation and were using kayaks and paddle boards hired from their hotel around noon when rough ocean and wind conditions started dragging them out to sea.

A search helicopter found the mom and two children wearing life jackets and clinging to a paddleboard at 8:30 p.m., police said. They had drifted 14 kilometres from Quindalup in Western Australia state, after spending up to 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,” Insp. James Bradley said.

‘One of the hardest decisions’

Joanne Appelbee told reporters on Tuesday she sent her oldest child for help because she could not leave the three children.

“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 confident he would reach shore but was filled with doubt as the sun set and help had not 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.

The three were all shivering and Beau had lost sensation in his legs because of the cold by the time they were rescued, the mom said.

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

All four family members were medically assessed but none required hospital admission.