UPDATED 1.49pm: A truckload of untreated human waste has been spilled onto an Auckland motorway today.

The Fire Service have now removed the spilled effluent that was being transported from the Auckland Sewage Treatment Plant.

Northern Fire Service shift manager Paul Radden said firefighters initially thought the load was wet concrete.

"It is untreated human waste from the Auckland Treatment Plant."

Crews were now attempting to wash the human waste down the motorway to a safe environment.

"We're liaising with Auckland Council Environment as to where we can contain it to."

The New Zealand Transport Agency is advising motorists to wash their vehicles if they passed through the effluent spill.

"We'd hate for anyone's vehicle to be damaged by this effluent, so our advice would be to use a garden hose to wash the paint and the undercarriage of your vehicle, and be careful not to use your bare hands," said acting Auckland Highway manager Mieszko Iwaskow.

All motorway lanes have reopened but traffic remains banked up.

Watercare spokesperson Rachel Hughes said there were no reports of accidents involving Watercare trucks or those of its contractors today.

"It doesn't appear to have anything to do with Watercare," she said.