NSW Fire and Rescue crews are working to clean up a diesel spill at Coffs Harbour Marina on Monday.
50-100 litres of diesel was spilled into the water at 12:30pm due to a refuelling mishap involving a yacht docked at the marina.
Authorities say the spill occurred due to a faulty fuel valve on the boat.
Coffs Harbour Fire Station commander Mick Kearns said although the spill was significant, crews were able to bring the damage under control.
"We used containment booms and absorbed the diesel with pads," Mr Kearns said.
"Luckily for us the wind was pushing in the right direction and we were able to prevent environmental damage to the marina."
The vessel operator is expected to face a penalty notice under the Protection of the Environment Operations Act 1997 of up to $7500 for polluting the waterway, in addition to all clean-up costs.
As of 6pm crews were still working at the site.