A guard dog at a Bacchus Marsh mechanic’s workshop was to blame for a mammoth oil spill into the Werribee River that took several days to clean up, the Environment Protection Authority has been told.

Traianon Transport was being investigated over the spill, which contaminated sections of the river at Peelmans Lane, Maddingley, earlier this year.

The Environment Protection Authority was told the only “plausible” explanation for the oil leak was that the company guard dog bumped open the waste oil tank valve, causing the spill.

The tank leaked about 1200 litres of oil, but EPA South West regional manager Carolyn Francis said it was unclear how much had entered the river.

“The oil travelled into a stormwater drain at the rear of the premises, which resulted in it flowing to a discharge point on Werribee River,” Ms Francis said.

“Traianon Transport was already cleaning up the spill on their site by the time EPA officers arrived and was co-operative throughout our investigation, but this leak was preventable.”

At the time, Melbourne Water put out booms to contain the oil and Moorabool council contractors removed the oil from the stormwater network and the river’s surface.

But there were serious concerns for native flora and fauna, with residents reporting a duck covered in black oil and fish surfacing where the oil had spilled.

Ms Francis said, thanks to prompt community reporting, authorities were able to respond quickly and minimise long-term environmental harm.

Moorabool Environment Group president Ben Courtice said while he was pleased damage had been minimised, three months on, oil residue is still stuck to reeds on the riverbed.

“Even if the dog, or a forklift, or a tractor knocked a tank of oil open, there should be measures in place so it’s not going into the river,” Mr Courtice said. “Hopefully, other companies will notice and take more care.”

The EPA issued Traianon Transport a notice requiring it to install controls to prevent future oil leaks, and fined the company $7700.

Traianon Transport declined to comment.