Police inquiries are continuing into how lactose was spilt for about 5km on State Highway 1 near Oamaru on Sunday night.
Senior Constable Graeme McPherson, of Oamaru, said police were called at 8.50pm after reports of lactose being spilt from a milk tanker on Alma-Maheno Rd, between Maheno township and Woolshed Rd in Reidston.
The tanker appeared to have a mechanical fault, he said.
The milk tanker was not owned by Fonterra but was contracted to it, he said.
No charges had been laid and the inquiries were continuing.
The spill had not caused any vehicles to crash and the highway was not closed because of it.
Firefighters were called to clean up the spill on Sunday.
The lactose - a type of natural sugar found in milk and dairy products - was visible on the highway yesterday, he said.
''I don't think it's a danger to motorists.''
