Austin Appelbee swam 4km 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, eight, 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.
"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."
The family, from 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 mother and two children wearing life jackets and clinging to a paddleboard at 8.30pm local time, police said. They had drifted 14km from Quindalup.
"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 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 the ABC.
She said she was confident he would reach shore but was filled with doubt as the sunset 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," Ms Appelbee said.
The three were all shivering and Beau had lost sensation in his legs because of the cold by the time they were rescued, she 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.
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