State labor inspectors found that a worker, who fell to his death this summer was using a safety line that was rusty and held together with duct tape.
The state labor department has cited Mary's Tree Service and Landscaping for four violations after completing its investigation into the workplace fatality.
The incident took place at Kaanapali Plantation on Maui on July 7, 2011.
Investigators said supervisors didn't understand their responsibility to ensure worker safety.
They failed to inspect equipment, failed to prohibit the use of defective equipment and failed to provide proper training for tree trimmers, according to investigators.
State labor officials say the company allowed 39-year-old Kelepi Latu to use a damaged safety line that was supposed to hold him in place.
"The line was broken and repaired with duct tape. There was rust, and with rust they should have known already, not to use the line," said deputy state labor director Audrey Hidano.
Two months ago in Kailua, 71-year-old Fifiti Misa was killed, when a coconut tree fell on him.
In January, a third tree trimmer died while on a job along Nuuanu-Pali Drive.
Labor officials plan to target the landscaping and tree trimming industry across the state, offering one-on-one safety training sessions with companies starting in February.