The leak in March 2006 came from a yard on Gateshead quayside used for washing vehicles. The drainage system was fitted with an oil interceptor but this was blocked and ineffective. The resulting slick stretched downstream to the mouth of the river Derwent.
Site owner Brett Oils has a discharge consent but no one at the company had thought about maintaining the interceptor after the operations manager was made redundant.
Brett Oils Business Services, the operator of the site and originally part of the same company, told the court it was responsible for checking the interceptor.
Appearing before Gateshead magistrates on 27 February, both companies pleaded guilty to pollution charges.
Brett Oils was fined £14,000 and £1,100 in costs for breaching its discharge consent, contrary to section 85(6) of the Water Resources Act 1991. Brett Oils Business Services received the same fine for causing polluting matter to enter controlled waters, contrary to sections 85(1) and (6) of the 1991 Act.