A record fine has been issued for a diesel spill at Hobart wharf.

Fishing company Australian Longline has been fined $40,000 and the captain of its boat, the Janas, received a $5,000 after 400 litres of diesel was spilled into the River Derwent in November 2015.

It tops the previous largest fine by over $35,000.

The Hobart Magistrates Court was told the spill created a 400-metre slick in an area known to be home to the endangered spotted handfish and Derwent sea star.

The spill occurred while the boat was refuelling.

Magistrate Simon Cooper found the crew had failed to put a bung into the tank designed to catch any spillage.

“I accept this offending is not the worst of its type, however a simple basic precaution could have prevented the offence for occurring in the first place,” Mr Cooper said.

“There's no evidence of actual environmental harm, but the potential of harm was present and is the reason for such legislation.”

The maximum penalty is $397,500 or four years jail for individuals and $1.59 million for a company.

“The penalties just show how seriously Parliament views offences of this type,” Mr Cooper said.

The court was told that Tasmanian-based fishing company regularly operated in some of the most environmentally-sensitive waters in the world.

Australian Longline has boats at Macquarie Island and Antarctica, and says it plans to expand into waters off South America, South Africa, New Zealand and the United States.

The company had the Janas on lease from New Zealand company Talley's to handle a doubling of its toothfish quota in 2015, so its crew was largely from New Zealand.

The court was told that Australian Longline had to pay tens of thousands of dollars to indemnify Talley's after the spill.