An oil spill in the Newcastle Harbour has cost one company over a million dollars.

The owners of the ship MV Magdalene will pay $1.2 million after 12 tonnes of heavy fuel oil were dumped into the harbour in August 2010.

The ship was conducting a routine de-ballast when it inadvertently discharge the dangerous contaminant.

The spill took nearly six weeks to mop up, affecting parts of the Newcastle harbour, nearby shorelines at Kooragang Basin corner, Walsh Point and the northern part of the Hunter River.

The New South Wales Land and Environment imposed the large fine on the owners this week, after a long investigation and prosecution by the Newcastle Port Corporation.

The $1.2 million fine is the largest amount ever charged for a breach of the state’s marine act.

The Newcastle Port Corporation said the decision will stand as a sign of the importance of marine habitats and the seriousness of their protection.