In a 7-0 decision, the judges say Castonguay Blasting Ltd. was rightly convicted under the provincial Environmental Protection Act.
The company was initially acquitted, but a higher court overturned that and the provincial appeal court upheld the conviction.
The company told the Labour and Transport departments about the 2007 incident, but not Environment.
The justices say the rock that flew from the blasting site along a highway set for widening was a contaminant, and the act requires immediate notification to the Environment Ministry.
The company argued that since the flying rocks didn't harm the environment, it did not need to report, but the justices said the act in question deserves a generous interpretation.
"Environmental protection is a complex subject matter — the environment itself and the wide range of activities which might harm it are not easily conducive to precise codification," Justice Rosalie Abella wrote for the court.
"As a result, environmental legislation embraces an expansive approach to ensure that it can adequately respond 'to a wide variety of environmentally harmful scenarios, including ones which might not have been foreseen by the drafters of the legislation.'"
She said the debris from the blast, known as "fly rock," met the act's definition of contaminant and should have been reported.
"There are two pre-conditions to this reporting requirement — the discharge must have been out of the normal course of events and it must have had — or was likely to have — an adverse environmental impact," she wrote.
"The purpose of the requirement is to let the ministry know about potential environmental damage so that any consequential remedial steps can be taken in a timely way."