Two separate spills on State Highways in Auckland have been cleared and congestion is easing.

Earlier this morning a police spokeswoman said an oil spill, or something similar, had caused three minor crashes near Whenuapai.

Although the crashes were minor, with no injury, the spill caused issues for drivers on the road, she said.

"Please take alternate routes or delay travel if possible until the spill is cleaned up by council, who have been advised."

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The spill has been checked by police units who attended and the road was now open.

NZ Transport Agency also reported an oil spill on the Southwestern Motorway.

The left lane was closed just before the Puhinui on-ramp as maintenance crew cleaned the road surface.

Motorists were advised to expect delays or to avoid the area if possible, however the spillage was cleared about 9.40am.