Richard John Hopley, who operates Hopley's Transport, pleaded guilty in the Land and Environment Court to one count of employing a person to transport dangerous goods without having a relevant licence and one of water pollution. The charges were brought by the Environment Protection Authority.

The pesticide spill occurred on February 8 last year at the depot Hopley used to temporarily store loads in Wetherill Park, in Sydney's south-west.

A driver employed by Hopley was unloading a 1000 litre container of pesticide when it fell off the forklift and split open. The pesticide was Farmoz Electra 225, a chemical used by cotton farmers to kill insects. In water its impact is toxic, killing fish within hours.

The contents drained onto the road and flowed into the stormwater system and Prospect Creek. The steep slope of the road and heavy rain meant the spill couldn't be contained.

EPA director-general Lisa Corbyn said the spill had a devastating impact on the waterway.

"The pesticide killed most, if not all the fish it came into contact with, including bream, mullet, eels, carp and a variety of native gudgeons. More than 3.4 tonnes of dead fish were recovered from the creek," she said.

Ms Corbyn said investigations revealed the employee involved did not hold a licence to transport dangerous goods. It had lapsed and he was in the process of applying for a new one.

The court found the driver had informed Hopley that he did not hold the appropriate licence and accordingly that Hopley had paid "scant regard" to his obligations.

NSW Land and Environment Court Justice Denis Cowdroy fined Hopley $15,000 for employing a driver without a relevant licence and $20,000 for polluting the creek.

He said the fines would have been higher if Hopley had not shown contrition.

"The only redeeming feature is that the defendant has pleaded guilty."

Outside the court, Hopley said he regretted the incident and was sorry.

Hopley was also ordered to pay the EPA's court costs, estimated at $15,000.