"We are very disappointed that this has occurred," he said.
The Auckland-based company - which is also responsible for maintaining the Auckland Harbour Bridge - was added to the drawn-out Taramakau job just before Christmas.
TBS Farnsworth took over from Greymouth company GT Liddell Contracting at the northern end of the bridge, although Liddells are still working on the southern end.
Mr Hutchinson said his company would be co-operating fully with the West Coast Regional Council as it investigates.
TBS was brought in by Kiwi Rail to coat the steel bridge. This involves sand-blasting using a material called garnet, which is very light and red in colour.
GT Liddell Contracting general manager Graham Liddell said his company was responsible for doing the steel repairs and replacing the pier caps, and it would finish at the bridge by the end of the month.
His team had received complaints about the dust that accumulated from the work being done at the other end, usually from motorists who said they could not see clearly as they drove across the bridge.
Under a resource consent for air discharge, TBS Farnsworth is not allowed to discharge contaminants that the enforcement officer deems dangerous or that may reach beyond the road and rail reserve that could have "an adverse effect on the environment".
The Greymouth Star this week provided the video footage to the council staff, who suggested that the material being dumped over the side of the bridge was grit that had accumulated over time in the scaffolding.
"One of the consequences of having scaffolding under the bridge is that it captures normal road dirt, sand and stones, which would normally fall through the cracks in the bridge to the river. This is the material which is being shovelled into the river by the workers on site, not the material used for the sand blasting," planning and environmental manager Michael Meehan said.
He said the council would now be working with the contractor to find a more suitable way to dispose of the accumulation of sand and dirt.
"We didn't think it was a very good look," Mr Meehan said.
Any rust that was removed from the underside of the bridge by sandblasting was recycled via a pipe to a dust bag, which was then filtered for re-use.
The resource consent allowed for incidental discharges within the area, which took into account that the nature of garnet meant that some escaped, he said.
