Environment Division staff are investigating a small oil spill from a bulk concentrate vessel berthed in the port of Burnie.
The crew of the Hong Kong registered Cape York noticed heavy fuel oil coming from their ship when it began discharging ballast water just before 8.30 on Monday morning.
"The crew acted immediately to shut down the pumping of the water which halted within a few minutes," senior environmental officer Rosemary Holness said.
"Approximately 100 litres of oil caused a slick about 10 metres by 30 metres."
Workers from TasPorts deployed a boom to contain the oil slick in the port area and used oil absorbent pads to soak up the spill.
No wildlife or other assets were affected by the spill.
"We were also fortunate that the weather played a positive role with easterly winds helping prevent the spread of the oil," Rosemary said.