Nearly 20,000 litres of oil from Whyte and Mackay's Invergordon plant spilled into the Cromarty Firth in the north-east of Scotland last August.
The company pled guilty to the offence under the Water Environment Regulations at Tain Sheriff Court yesterday.
The leak, from a storage tank and a crack in its surrounding wall, was found by the Scottish Environment Protection Agency (Sepa). Officers concluded that the spill could have been avoided by adequate monitoring and maintenance of the tank and wall.
Craig Harris, head of the wildlife and environment crime unit at the Crown Office, said the spillage was "entirely preventable".
"Industry must ensure the proper maintenance of their systems to ensure they meet their environmental obligations," he said.
"There could have been grave consequences if the leak had gone undetected or if it had occurred during the breeding season."
The Cromarty Firth, which leads into the Moray Firth, has been designated as a site of special scientific interest and a special protection area, and supports breeding birds such as osprey, whooper swan and greylag goose.
David Wilby, Sepa investigating officer, said: "An alarm on this tank would have notified the distillery of a spill and would likely have resulted in this spill being contained."
A spokesman for Whyte and Mackay said: "We accept full responsibility and apologise profusely for the incident.
"As a result of the incident, we are investing significantly in the infrastructure to ensure this does not happen again."