When Amber Bourke bobbed to the ocean's surface off the Philippines, she thought she'd broken a freediving world record. Then she lost consciousness.

The blackout only lasted seconds — a blink-and-you'll-miss-it moment — but the momentary lapse was enough to disqualify the Brisbane swimmer's near-perfect 73-metre dive.

She spent a year training in the warm, calm waters off the coasts of Indonesia and the Philippines to break the 72-metre no-fins freedive record held by Japan's Sayuri Kinoshita.

The dangerous sport tests how long a swimmer can dive or swim on a single breath.

She trained all over the world to prepare for the world record attempt. ( Facebook: Amber Bourke )

Speaking to ABC Radio Brisbane's Breakfast program with Craig Zonca, Ms Bourke said her record attempt was a culmination of a lifetime of training.

"I was actually on the Australian representative synchronised swimming team throughout high school and after that I discovered freediving," she said.

"I could hold my breath for three minutes and I thought that was amazing.

"Then I discovered freediving and it was a whole other level."

Drifting into the darkness deep below the ocean's surface, without the security of an oxygen tank or flippers, is Ms Bourke's specialty.

She said the descent and return to the surface were physical challenges masked by a kind of controlled calm.

"Your arms are burning, your lungs are burning, all you want to do is take that first breath.

"You have to stay as relaxed as possible and as efficient as possible."

Amber can hold her breath under water for six minutes. ( Supplied: Janmi Wong )

A video of her six-minute world record attempt shows her tethered to a guide line with her eyes closed and cheeks bulging with air.

She said the pressure 70 metres under water compressed any oxygen in her lungs, so holding a reserve of air in her mouth to regularly equalise was a must.

"You need to be completely relaxed to be able to withstand that pressure and stay in the moment during the dive.

"Anything that improves your flexibility is very helpful and meditation to be able to slip into that very deep relaxed state."

Amber started freediving after representing Australia in synchronised swimming. ( Supplied: Janmi Wong )

Australian authorities raised concerns about the popularity of the sport in April after qualified scuba diving instructor Dmitriy Ross died while freediving off a Sydney beach.

His family told the ABC they suspected he lost consciousness in shallow waters while wearing a weight belt, making it impossible for him to resurface once he blacked out.

Despite the dangers, and her recent blackout, Ms Bourke said she still had her sights set on breaking the world record.

"I got really close on this last attempt and I'm really keen to give it another shot."