Questions have been raised over what causes sinkholes.

What locals have described as a sinkhole has formed over a residential road, directly over the M4 Tunnel.

Sydney Water says the 10m wide hole at Burwood is a road subsidence.

Engineers Australia Geotechnical Society Professor David Airey told Joe Hildebrand a road subsidence is what occurs before the ground collapses into the hole.

“A sinkhole actually is a hole … and they can go quite deep.

“You do get holes associated with water main bursts, that’s probably the most common issue.”

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Image: Nine