Walker, a silver medallist in the under 23 road race at the 2005 UCI World Championships in Spain, made a premature exit from the sport in 2009 due to tachycardia, and earlier this year announced he was returning to racing with the Melbourne-based Continental outfit.
In his second race of the Australian National Road Series, the Tour of Gippsland earlier this month, Walker won the eighth stage en-route to the overall victory.
Doubled up on the side of the road on Wednesday on the afternoon stage two, Walker was distressed. He got back on his bike and waved off teammates who went to the back of the bunch to assist and it was then announced that he would not continue.
According to a press release sent out late on Wednesday evening by Drapac, Walker "experienced a light episode of tachycardia... a condition he is known to suffer from, and chose to retire from the race and seek medical attention."
Walker was then sent to the Portland Hospital for observation and tests.
"He returned a clean bill of health and was later discharged," the release continued.
Today, Walker will return to Melbourne for further medical consultation.
"William made the right decision to retire from the race and return to Melbourne for further assessment," said team manager Jonathan Breekveldt. "The health and wellbeing of our riders is paramount and Will has the full support of the team and its resources to return him to full health."
The 26-year-old spoke to Cyclingnews following the announcement in February of his return, explaining that a range of testing including regular electrocardiograms (ECGs), stress tests and echocardiograms indicated that he was back to full health.
"I just started doing some rides with my brothers [fellow cyclists Johnnie and Nick] and locally and I was going quite well off very little training and my heart was feeling good again and I just thought at 26, it was a good time to get back into it again and give it one last chance," Walker, who rode with Rabobank and Fuji Servetto in his previous racing life, said.