A contractor's error is believed to have been responsible for a fuel spill that dumped thousands of litres of diesel at the D'Albora Marinas at Rushcutters Bay.

Authorities, meanwhile, are also investigating a pollution slick in Rose Bay to the east to determine whether it is sourced from a separate spill.

The d'Albora Marinas portfolio comprises seven high-profile marinas.

Sydney's Port Authority and the NSW Environment Protection Authority are working with the marina to contain and remove the spill using floating booms and other equipment, with investigations on the cause continuing.

Tim Allerton, a spokesman for D'Albora said "close to 6000 litres" spilled from a diesel tank, with contractors alerting authorities of the spill at about 8pm on Tuesday. The Port Authority put the size of the spill at about 7000 litres.