Mr Dall said he did not believe his staff member was carrying the bleach used in the assault on State Highway 10 south of Kerikeri about 7am on Monday.
The man had pulled over after the car behind him repeatedly flashed its lights.
When he stopped at least two men held a knife to the side of his face leaving him with cuts, and threw bleach into his eyes and face. They took his wallet, which they discarded at the scene, and cellphone before continuing south towards Pakaraka.
A local found the injured man in a distressed state and called emergency services.
Mr Dall, a senior monitoring officer at the council, did not want to comment on any possible motive for the attack, but said police were still appealing for information and "exploring a number of possibilities around the motive".
Police had assured him, however, there was no known risk to any other council staff member.
Despite the seriousness of the attack, the effects did not appear to be too bad, council chairman Craig Brown said.
The council had no indication whether the attack was random or targeted.
"We don't know of anything in particular that would point to something like this happening, but I dare say police will question staff members about that," Mr Brown said.
"Management have been asked to think of anything at all ... but we're at a loss. To us it appears to have come out of the blue."
Apart from urging staff not to stop for people they did not know, or to do so only where others were around, Mr Brown said it was hard to know what precautions employees could take.
The staff member is part of the regional council's monitoring team, which is responsible for making sure resource consent conditions are adhered to.
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