Auckland Council has explained what's causing the sinkhole on College Hill Road.

The metre-by-three-metre hole opened up yesterday afternoon, disrupting traffic between Ponsonby and the CBD.

Head of Healthy Waters Andrew Chin says there's a clay stormwater pipe over a hundred years old which has a hole in it.

He says the surrounding fill is being washed into that hole.

"Once the road seal breaks, the process accelerates and that's where all the fill's going, it's going down into the stormwater pipe. It's created this sinkhole." 

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