The Honda which speared off a low bridge and crashed upside down into a storm drain in Wanniassa on Australia Day had driven 100 metres along the median strip before the fatal incident.
The driver, who was the sole occupant of the car, received significant injuries in the crash and was attended by people who saw it happen. He later died at the scene. Police have not disclosed his age and suburb.
Police are yet to take formal statements from the witnesses but are appealing for any others who were on Drakeford Drive just before 8pm on Wednesday night and may be able to assist the investigation to come come forward.
The ACT-registered Honda was travelling northbound between Taverner St and Atthlon Drive when it drove onto the median strip for a lengthy time, returned to the roadway, crossed the dual carriageway and drove off the low concrete bridge, flipping onto its roof.
The driver's death was the second road-related fatality recorded in the ACT this year after a cyclist hit a tree on Black Mountain Peninsula four days ago.
Police crash investigators admitted the circumstances of the Tuggeranong incident were most unusual and were at the scene well into the early hours of Thursday morning trying to determine why it had occurred.
The Honda was not retrieved until late Thursday morning. It has been seized and is being mechanically assessed by investigators.
The investigation is likely to continue for a week.
Witnesses reported hearing loud bangs at the time of the incident, and distressed observers watched as emergency service crews tried to access inside the car. A witness said emergency services had used a ladder to get out of the drain.
The northbound lanes on Drakeford Drive from Taverner Street to Athllon Drive were closed overnight. The car was removed from the scene on Thursday morning.
ACT Policing's major collision team is preparing a report for the Coroner.
