The fuel tank came as a surprise to contractors from Tru-Line Civil when it was revealed buried two metres down.
It had a fuel smell to it and excavation revealed a small access hole to the tank.
Ashburton council infrastructure services manager Neil McCann said a cautious approach had been taken with the tank discovery.
Between 400 and 500 litres of fuel had been sucked out of it on Tuesday and the access hole then sealed.
The fuel tank is buried in front of a building once used by Mid Canterbury Transport.
The company took over 104 Moore Street in 1940 from Poore Motors and used it to service vehicles and also as a depot for taxi and passenger service buses it ran.
Petrol pumps were installed and fuel was available to the public, but was mainly for company use.
The depot was closed when operations were transferred to a new site at West Street near Mill Creek in 1960.