High levels of blue green algae in Lake Burley Griffin's central basin have forced triathlon organisers to scrap a swim leg and re-badge the event as a "dryathlon".
Canberra Triathlon Festival event director Mark Emerton says health concerns have made it impossible to proceed with the swim leg of the event scheduled for Saturday.
The race will now revert to a duathlon format, with competitors now set to run, ride and run again.
"We put the health and safety of our competitors as our highest priority and this isn’t the first time the National Capital Authority has been forced to close the lake, so we are always prepared to switch to Plan B," Emerton said.
"For the popular corporate and community triathlon event, the 200-metre swim will be replaced by a one kilometre run.
"I know there’ll be disappointment for those involved in the swim-leg but the weekend forecast looks perfect and we’re still looking forward to a great, fun day, especially for those who have signed up for their first event of this type.
"With the lake off-limits we’ll now be calling the event a 'dryathlon'."
Online registrations for the event will close at 5pm on Wednesday however participants can still sign up at the event site on Saturday morning.