THe mess was discovered in Warbrook Road, Melaleuca, in the Gnangara pine forest and reported it to DER.
Acting director-general Jason Banks said the container of oil could have been dumped there up to three weeks ago.
He said the maximum penalty for illegal dumping was $62,500 for individuals and $125,000 for companies.
City of Swan acting chief executive Colin Cameron said it took council staff two days to remove the oil waste and dog out contaminated soil.
‘The City will have to pay $4000 to dispose of the soil,’ he said.
‘However, we were fortunate it had not seeped deep enough to impact upon groundwater.’
Ellenbrook has been the subject of 17 major illegal dumping incidents since January, according to Keep Australia Beautiful WA.
A KABWA spokeswoman said Ellenbrook bordered State Forest 65. more commonly known as Gnangara pines.
‘State forest 65 is a considerable area, comprising the suburbs of Gnangara, Nowergup, Jandabup, Neerabup, Pinjar and Mariginiup,’ she said.
‘Since January 1, there have been 17 reports of illegal dumping in the pines.’
That figure does not include any dumping reports made to local governments.
Mr Cameron said the City was keen to hear from anyone who may have information relating to the dumping incident.
‘Aerial imaging shows the dumping took place after November 9 , so I ask community members to cast their minds back and, if they recall anything that may be important, to contact us,’ Mr Cameron said
‘The fines for people convicted of illegal dumping have are severe.
‘We hope these penalties, a considerable rise from the previous maximum fine of $1000, will act as a deterrent.’
The City of Swan has also commissioned a new mobile CCTV unit that will be used to target dumping hotspots.