VicRoads believes the large crack in Melbourne's WestGate Bridge is heat related but says drivers can have total confidence in the bridge's structure.
The crack, about five metres in length, appeared on Monday morning on the bitumen surface of a steel section of the bridge.
VicRoads acting chief executive Peter Todd said engineers immediately climbed into the bridge and determined the structure was sound.
He likened the crack in the road to a carpet tear in a house.
"We believe there's been a separation of the asphalt from the membrane underneath," he said.
"It's a bit like getting a tear in your carpet, the floor underneath is still sound.
"I can say the bridge is sound and motorists should have full confidence."
VicRoads believe last week's record heatwave, where temperatures reached more than 41C for four days in a row, caused the damage.
The crack is in the inbound lanes one and two of the bridge, which temporarily closed in the afternoon for emergency roadworks.
Victorian Opposition Leader Daniel Andrews says the damage was evidence that the bridge, the only direct freeway connection between the city and its western suburbs, was overworked.
"What we know without any doubt is if you can get 5000 trucks every single day off the West Gate Bridge that's a good thing for safety, it's a good thing for the longevity of the West Gate Bridge," he said.
All five lanes of the bridge reopened for the afternoon peak hour, but three lanes will close overnight as crews complete a more thorough repair job.
"We don't expect any more cracks on there, but of course we want to know why this happened," Mr Todd said.
"We'll be having some specialists go with the workers tonight to look at it."
The bridge will be reopened in time for Tuesday morning peak hour traffic.
The West Gate was recently closed overnight for four nights while scheduled resurfacing works took place.