Whangārei road fissure cause unknown

The hole on the surface, with a long fissure underneath. Photo / Michael Cunningham

Roading experts are still scratching their heads over why a fissure has opened up in a Whangārei street.

Water scouring could be what caused a hole four metres long and up to one metre deep to develop under the road surface in Norfolk St, on the edge of the city centre, last Wednesday.

Witnesses say the half metre diameter hole on the road surface does not indicate the bigger cavity underneath.

Norfolk St in Whangārei is still reduced to one lane because of a sinkhole which developed last Wednesday. Photo / John Stone

However, despite the ''water scouring'' theory, the hole is dry with is no water lying in it or sign of where any might have moved through. A camera probe of the hole and a sewer line and which runs under it has shed no light on why the road's sub-surface caved in.

''The cause of this cavity remains unknown at this stage. CCTV camera investigation of the sewer line which runs directly under the cavity has shown no obvious cause,'' a Whangārei District Council roading representative said.

''It appears that water scouring is the most likely cause.''

Further investigations are continuing to establish why the hole developed and ensure any underlying issues are addressed to prevent a recurrence, he said.

The northbound lane on the street - which runs parallel to Bank St - was closed on the day of the incident, with a wire cage around the surface hole. Traffic management and cordons will remain in place until investigations are completed and the road reinstated.