Thieves are thought to be responsible for approx 300 litres of heating oil fuel to be leaked from a tank at a community centre at Penmorna on the 3rd March.
Damage was caused to a pipe and a valve was found to be open.
Police are urging home owners to be extra vigilant and to make sure that their tanks were secure.
Firefighters from Porthmadog were called to the scene. The oil is believed to have flowed into domestic drains and a water course. There was no risk to members of the public.
Firefighters succeeded in stopping the flow and used environmental spill packs to assist with their work.