Wollongong Local Court heard Sebastiano Russo was driving the DAF tilt tray truck between Smithfield and Dapto with a front-end loader and attached backhoe on board when the machine clipped the overpass, causing a sizeable gouge in the concrete.

The impact of the crash caused the backhoe to partially break away from the loader and come to rest on the vehicle's tray, held on only by its hydraulic lines.

Road and Maritime Services (RMS) documents presented to the court said the combined height of the vehicle and the machinery measured 4.5 metres after the crash - 100 millimetres short of the 4.6-metre clearance height of the bridge.

However, RMS lawyer Brad Glennen said obviously the height of the truck and backhoe before the impact had been greater than 4.6 metres to cause the collision.

When officers interviewed Russo at the scene, he admitted knowing his maximum allowable height had been 4.3 metres, but he conceded he had not measured the vehicle and the load before starting his journey.

"I was doing a favour for a mate; I thought it was going to be OK," he said.

Russo was charged with a severe risk breach of allowable dimension limits.

Presiding magistrate Michael Stoddart noted the incident occurred at 4.25pm, at the start of peak hour.

"It's probably luck there wasn't more damage to the bridge or to other motorists."

Russo was fined $5000. The 55-year-old did not attend court to hear the sentence.