NUTS AND BOLTS

Opened in 1978. 462 metres long, 12.8 metres wide and 7.9 metres high. More than 30km of cable used in upgrade. 1120 high-pressure sodium and fluorescent lights used. 140,000 hours of work doing upgrade. More than 40,000 cars a day use the tunnel.

‘‘We just basically kept the hole and replaced everything else in the interior of the tunnel,’’ Rod James, the agency’s Wellington state highway manager, said.

Perhaps the most obvious safety features are the 770 fireresistant orange wall panels to help prevent the spread of fire and protect crucial utilities behind.

Eight fans – each with the power of a small car – replace the 18 old fans in the tunnel.

Two of the new fans have more power than all the old fans combined.

During day-to-day operations, the fans help control air quality but in a fire they can rev up to pump out air at 100kmh to fight Wellington’s wind and pump smoke from the tunnel.

Thirteen cameras have been installed in and around the tunnel – to monitor traffic and driving but also keep an eye out for smoke.

Some of the cameras can rotate and zoom so controllers can get a close look at problems and direct emergency services.

Mr James says the leaking roof was intentional with ‘‘weep holes’’ built in. ‘‘If we didn’t do this, pressure would build up and could damage the tunnel lining.’’