Around 100 litres of diesel has spilled into the Motueka Harbour and estuary.
The diesel, from nearby Port Motueka, is covering a very large area but is not thought to be threatening marine life or shellfish.
Residents noticed a strong smell of diesel in the area yesterday.
Tasman District Council spokesman Chris Choat said officials have decided to rely on natural forces to disperse the diesel.
An investigation will be undertaken into the source of the spill.
