An excavator, being towed by a truck, is believed to have struck the underside of both carriageways of the Commonwealth Avenue Bridge, causing debris and damage.

Police say the truck was travelling westbound on Parkes Way, under the Commonwealth Avenue flyover, when it struck the bridge at about 3.35pm on Tuesday.

The incident caused significant traffic delays, with one westbound lane on Parkes Way closed for more than six hours due to debris and to allow engineers to examine the damage.

Police want to speak to anyone who witnessed or has dash-cam footage of the incident.

Call 1800 333000, or via the Crime Stoppers ACT website.