It happened around Rockaway Boulevard in Rosedale just after 11 a.m. near the Nassau County border.
Officials said the 18-wheeler was leaking about 2,000 gallons of fuel into the roadway, storm drains and the canal leading to other channels.
The leak has now been stopped and foam is covering all of the leaked fuel, officials say.
Officials say the tanker has a 12,400 gallon capacity but was only carrying 8,500 gallons at the time of the incident.
Another truck will offload the gas off the overturned vehicle, which will take some time.
The truck driver and two others were checked out on the scene.
Officials say 100 firefighters from 17 volunteer fire departments in Nassau County are on the scene along with the FDNY.
Nassau County police will investigate how the truck overturned.
One witness said the driver had just left the gas station and flipped while turning onto Rockaway Boulevard
Crews say it will take hours to clean up the spilled fuel.