"Stumpmaster had extendable barriers and tape available to mark out an exclusion zone but chose not to use it," said WorkSafe's deputy general manager, investigations and specialist services Simon Humphries.
"As a result, an innocent party has received traumatic injuries," Humphries said.
"You are responsible for ensuring that other people are not put at risk by the work of your business. This includes members of the public who could be affected by a work activity".
Stumpmaster was sentenced under the Health and Safety at Work Act 2015.
The maximum penalty was a fine not exceeding $1.5 million.