The car, a Lexus SUV, was attempting to navigate around some sandbags at 2mph before it hit a bus, which was travelling at 15 mph, according to a report from California’s Department of Motor Vehicles.
The report claimed that the vehicle and its test driver "believed the bus would slow or allow the Google vehicle to continue". No one was injured in the crash.
Google has until now remained quiet on the issue, but has since said its vehicles are now "learning" to be wary of bus drivers.
In a statement to CNBC, Google said: "In this case, we clearly bear some responsibility, because if our car hadn’t moved there wouldn’t have been a collision."
"From now on, our cars will more deeply understand that buses (and other large vehicles) are less likely to yield to us than other types of vehicles, and we hope to handle situations like this more gracefully in the future."
The news comes three years after the leader of Google's autonomous-car project Chris Urmson claimed that the firm's driverless Lexus and Prius models are safer than if a human drove them.
Other Google cars have been involved in incidents, but these were put down to human error. This is the first time the company has admitted any fault.
Google aims to have driverless cars road-ready by 2020.