It’s understood the man was working at the Harvey Street site in Mosman Park when he fell about three metres into the trench that was filled with dirt and water.
His colleagues tried desperately to save him before making an emergency call at 2.30pm on Thursday.
Firefighters and an urban search-and-rescue team worked for hours, but sadly he could not be saved.
A pump was brought in to try and remove some of the water, so the man’s body could be retrieved.
Police and Worksafe will now launch an investigation into how the fatal accident unfolded.
It is the second workplace death in WA this week after a tree lopper, aged in his 40s, was struck by a branch while in a cherry picker in Waroona on Tuesday.