This week's Hume Highway closure was the longest seen in a decade, according to Camden Local Area Command Traffic Services Senior Constable John Kane.

Sen Const Kane said the highway was closed for around 55 hours when a b-double truck carrying "dangerous" chemicals overturned near the Mittagong exit on Monday.

The accident saw all four lanes closed on the highway with traffic diverted through Mittagong along the Old Hume Highway.

"It's one of the longest closures I've seen since in the last ten years," Sen Const Kane said.

Sen Const Kane said the reason why the highway was closed for such a lengthy period of time was because the Environment Protection Authority (EPA) wanted to make sure the chemicals were cleaned up completely.

"The EPA wanted to ensure they cleaned up as best possible so there was no seepage into water systems and it took much longer than they expected," he said.

"The chemicals were dangerous not only to people but the environment as well."

Sen Const Kane said police had received some complaints from residents in Mittagong after traffic was diverted through the town.

"We had some complaints about the noise of heavy vehicles," he said.

"It's unfortunate it had to be done but I think people would prefer to know that the chemicals were cleaned up so there were no long term affects."

Police were assisted by the Roads and Traffic Authority, NSW Fire Brigade Hazard Materials Unit and the Environment Protection Authority.

Cyanide spill ruled out by authorities

The Environment Protection Authority (EPA) has ruled out cyanide as one of the chemicals spilt in Monday's truck accident which closed a section of the Hume Highway for more than 40 hours. EPA officer Liane Sayer-Roberts said resin solution, adhesive and diesel were detected in the samples taken from the site.

Ms Roberts said the EPA left the site on Wednesday but the Roads and Traffic Authority along with the Sydney Catchment Authority remained on site to clean up existing residue.

"It was rather fortunate circumstances and swift action by the emergency services," Ms Roberts said.

"It was fortunate that the amount in the small section of the creek bed that the chemicals spilled into was dry and no environmental damage was done."