Australian surfing has a new world champion — and the teenager only got out of hospital in December.

Kai Colless sealed the world title in his division after winning the final of the US Adaptive Surfing Championships in California at the weekend.

The 16-year-old felt he put in a solid performance, but it wasn't until the ocean seemed to go flat at the end of his heat that he knew he'd won the final in the unassisted prone men's division — and the world title.

"When [the announcer] started counting down and there was no waves coming, that's when I really knew I had it," Kai said.

World champ Kai under the Huntington pier (Photo Brooke Colless)

His win came almost a year to the day after he started to experience the symptoms of a condition called arteriovenous malformation (AVM), while on holiday with his family in France. 

The condition impacted the arteries and veins in his spine, causing them to swell, which limited the use of his legs. 

The Gold Coast high school student said he had been working on gaining more leg function ever since. 

"So I can walk with crutches at the moment, I just … fall over a little bit, so I'm not super trusted … but working up to it," he said.

Family carries Kai to victory

Kai's mum, Brooke Colless, was there to carry her son up the beach on the weekend. She's also been there every step of Kai's way back into the water. 

"It was pretty scary, it's been a journey to say the least," she said. "We were worried about Kai and his legs atrophying."

Kai Colless was chaired up the beach by his mum (Instagram)

She said the family set a goal to get Kai back into the water in May, using a trip to Hawaii as motivation.

"He loves surfing, he loves everything water and the ocean, and we gave him the goal as he got back in and got back surfing that we'd take him to Hawaii," she said.

Kai won his first adaptive surfing event while there. When the family returned home to the Gold Coast, they received sponsorship travel to the second leg of the world adaptive surfing titles in Costa Rica, where he put in another good performance. 

The weekend's victory meant he secured the world title.

Months of hard work

Ms Colless said Kai had put in a lot of hard work since leaving hospital in December  "Nerves grow and repair at a very, very slow rate. So we've just been very patient and working super hard," she said. 

Athletic achievements aside, Ms Colless said Kai was making progress in his leg function.

"He's back up walking with a walking frame and learning to walk with crutches at the moment," she said. "He's still in a wheelchair 24/7 now, it's just when he's at the gym [that] he's re-learning to walk."

// BERN YOUNG, JESSICA LAMB, and NICHOLAS MCELROY
© Australian Broadcasting Corporation. All rights reserved.