Unleaded fuel spilled into drains and the sewer in Wandin North.
The Environment Protection Authority (EPA) is investigating the source of the spill, which occurred at 361 Warburton Highway about 1pm on Friday 30 November.
The Yarra Valley Water clean-up operation was expected to take a number of hours.
The authority tested sewer pipes and drains between Wandin and the Launching Place pumping station.
A CFA hazmat crew undertook atmospheric monitoring of houses and open drains in the immediate area.
Police asked people to avoid Sebire Avenue and surrounds, and advised against using naked flames within an exclusion area.
The EPA at the time said people might be exposed to petrol by smelling or breathing in the vapour.
“Petrol can be smelt at very low concentrations that are unlikely to be associated with adverse health effects,” they said.
“However, even at low concentrations in the air, inhalation of these vapours may cause irritation of the eyes, nose or throat, or even nausea depending on the person inhaling the vapour.
“At higher concentrations, dizziness, confusion or feeling unsteady on your feet may occur.”
The EPA said these side-effects should stop once the person moved away from the exposure, but they should contact a doctor if their symptoms continued.
