Two people were caught on CCTV running away from the Mt Victoria bus tunnel after a lump of wood fell onto a bus.

Richard MacLean, Wellington City Council spokesperson, told NZME the quality of the video is not good enough to determine if the wood was dropped on purpose – but it definitely shows two figures running away after the incident.

"It sounds possibly like a deliberate act," he said.

Wellington City Council staff inspected the slope above the tunnel and found no sign of a broken tree or slip.

"So there is a theory that it might have actually been a deliberate act by someone throwing some wood or timber or whatever down on to the front of the bus," MacLean said.

The tunnel was closed overnight on Tuesday afternoon after the wood struck the front of a bus, damaging the windscreen.

It was initially thought a section of the tunnel's roof had fallen.

Commuter buses to and from the eastern suburbs and Wellington Airport were diverted through the main Mt Victoria tunnel during the closure, and the tunnel reopened the following day.

MacLean says the council has not gone to the police yet and likely will not, unless it receives good information about the identities of the people above the tunnel.

"I think unless we start getting phone calls from people who know who was involved then we might not want to be in a position which might waste police time."

He added this appears to be an isolated incident and unless people start to gather above the tunnel for "nefarious means" the council is unlikely to pursue the incident.