About 15,000 litres of lactose, a dairy byproduct, was spilled near the Wairoa River south of Tauranga on Monday afternoon.
A tanker travelling to Hamilton lost part of its load when the edge of the road gave way and it rolled into a drain.
The Bay of Plenty Regional Council has been at the scene cleaning up.
Pollution prevention team leader John Morris said a lactose spill has the potential to kill fish in the waterway. But he said the river is not discoloured, so risk of that is low.
The tanker was removed from the site on Monday night and crews are now cleaning up the residue.
