Mt Wellington Highway is not closed, but traffic attempting to turn into Waipuna "will no doubt cause backlogs", police said.
Signage would be put in place to warn drivers.
The spill occurred around 4.50am, Fire Service northern communications shift manager Jarron Phillips said.
It was not yet clear what caused the spill, but the truck had not been involved in a crash, he said.
Around 20 litres of a liquid dye had been spilled over a few hundred metres of road.
No injuries had been reported, but the truck driver had been de-contaminated.
A road clean up crew had also been de-contaminated, Mr Phillips said.
Two Fire Service hazardous materials units were at the scene, where a full de-contamination tent has been set up, along with another three fire crews to deal with the spill.
It was expected to take "some time", he said.