A 13-year-old boy has been praised for his determination after he swam through rough waters in fading light to save his mother and two siblings who had been swept out to sea off Australia's coast.

Axar.az informs, citing BBC news, the family had been paddleboarding and kayaking at Geographe Bay in the south of Western Australia, on Friday when strong winds pushed their inflatables off course, police said.

The teenager began to paddle back to shore to raise the alarm but his kayak took on water - so he swam the remaining 4km (2 nautical miles).

"The bravery, strength and courage shown by this family were extraordinary, especially the young fella who swam 4km to raise the alarm," Naturaliste Volunteer Marine Rescue Group said.

The organisation's commander Paul Bresland described the efforts of the teenager as "superhuman".

"He swam in, he reckons, the first two hours with a life jacket on," he told ABC News. "The brave fella thought he's not going to make it with a life jacket on, so he ditched it, and he swam the next two hours without a life jacket."

The boy was able to raise the alarm by 18:00 local time (10:00 GMT) on Friday evening, sparking a huge search for the missing relatives from Quindalup beach, near Busselton, WA Police said in a statement on Monday.

The boy's 47-year-old mother, her other son, 12, and daughter, eight, were located at around 20:30 by a rescue helicopter clinging to a paddleboard about 14km offshore.

"A volunteer marine rescue vessel was directed to their location and all three were successfully rescued and returned to shore," the force said.