Traffic is crawling on both main roads into Wellington this morning, after a truck crash in the Ngauranga gorge.

State Highway 1 is down to just one lane southbound through the gorge, and traffic is backed up all the way past Porirua as a result.

Motorists are encouraged to use alternative routes into the city, including State Highway 58 and Lower Hutt.

However, the extra traffic there is also causing congestion.

NZTA's Mark Owen says the best option is to avoid the roads wherever possible.

"If people can take the train or semi-delay their journey at the moment they will have significant delays.

"So if they can avoid that area, or re jig their day."

Owen says the Johnsonville on-ramp has also been closed to ease pressure.

"What they're trying to do is look at diversion options, whether we can actually use some of the local road network to divert traffic just to try and free it up and utilise what options we have."

Police communications shift commander Guy Callahan says the truck is blocking the inner lane southbound just before the Newlands Bridge.

"There's enough room on the outside lane for traffic to get around at the moment, but I guess it'll cause delays while we get that vehicle off the road."

Guy Callahan says the driver of the truck has escaped unharmed.