AAP

No toxic material has been found on the stretch of the Pacific Highway where road workers fell sick after unearthing an unknown substance, Roads and Maritime Services (RMS) has confirmed.

However, the authority will continue to test the site before issuing the all-clear.

Road workers experienced nausea and vomiting after digging up an unknown, clay-like substance from a section of the highway near Port Macquarie just before Easter, it was reported on Wednesday.

They were taken to a doctor and have since recovered.

Independent testing on samples from the site had ruled out a range of contaminants, the RMS said on Thursday afternoon.

"Environmental and contamination assessment reports have today shown the highly oxidisable clay from the site contained no contamination from pesticides, polyaromatic hydrocarbons, petroleum hydrocarbons, polychlorinated biphenyls or any of the volatile organic hydrocarbons," the RMS said in a statement.

The RMS tests backed up an investigation on Wednesday that revealed no signs of radioactive material.

Work at the site remains on hold, and an exclusion zone is still in place as experts work to determine the nature of the substance found by workers, a spokesman told AAP.

It has been reported that chemical waste was buried at the site more than 30 years ago after an accident involving a truck carrying radioactive isotopes from Sydney to Brisbane.

On Thursday, NSW Roads Minister Duncan Gay appointed Sydney Water director Brian Gilligan to lead an independent inquiry into the suspected contamination of the highway.

"This investigation will include examining the approvals process for the work at the site, the risk assessments that were undertaken and what measures were put in place to protect road workers and the public," Mr Gay said in a statement.

NSW Premier Barry O'Farrell told reporters that waste appeared to have been buried and then forgotten at the upgrade site.

"I'm appalled that on a project that is meant to save lives by duplicating the Pacific Highway, lives were put at risk in such a cavalier fashion by the exposure of this deposit," he said.

But Opposition Leader John Robertson claims the government has been aware of the existence of chemical waste at the site "all along", and that tender documents for the upgrade project acknowledged its presence.

"Duncan Gay cannot pretend he did not know," Mr Robertson told journalists on Thursday, adding that the minister had "failed in his duty of care".

The government should have closely monitored weekly updates from the upgrade site, which would have meant there was "no need" for workers to be exposed to the unknown material, he said.

The RMS has advised there are no threats to the travelling public, and personnel are on hand to ensure the site is kept secure.