Reigning Olympic champion Scott Brennan has withdrawn from the national rowing selection trials, starting on Friday, to continue his hiatus from the sport.

A trainee doctor, Brennan, who won gold in the double sculls with David Crawshay in Beijing, was due to make his competitive return to sculling in the trials at Penrith.

But the 27-year-old doctor has opted to wait another year before beginning his assault to defend his Olympic title with Crawshay in London in 2012.

Brennan is currently a resident medical officer at Canberra Hospital and made the "tough decision" last week not to risk burning himself out by throwing himself back into rowing while also working long shifts.

The Tasmanian last rowed competitively at Beijing and took time off to finish his medical internship in 2009.

He intended competing at the 2010 World Cup and world championship regattas but eventually decided "hammering" himself would be counter-productive to his long-term Olympic hopes.

"It was a very difficult decision to make because it's in my nature to be always up there and racing hard," he told AAP.

"I just don't know how (smart) it is to maintain that intensity to be good enough to stay at the top for three years.

"It's my personal hope that if all goes well and we can put down the performances that we can give it a shot at winnings the double sculls again in London."

The decision means Victorian Crawshay, who raced in the quad sculls at last year's world championships, will likely row in the priority sculls boat in 2010.

Crawshay will be among the scullers beginning the selection trials at the Sydney International Rowing Centre on Friday along with another Beijing champion in Duncan Free.

Men's pair gold medallist Free will dust the cobwebs out in the sculls but will trial for a place in the fours or eights for the World Cup regattas in Europe.

With partner Drew Ginn currently focussing on cycling, Free had formed a new combination with fellow Queenslander Sam Conrad but Conrad has suffered an untimely injury.

AAP