One person is dead in a serious crash that has closed both directions of Wellington's Terrace Tunnel - the main route in and out of the capital's CBD.

The tunnel closure is causing major delays, with citybound traffic backing up beyond The Terrace offramp.

A Wellington Free Ambulance spokesperson said there were two patients, one in a critical condition and one with minor injuries.

However police have since confirmed one person died in the crash.

Emergency services responded to a crash involving a Crown removal van towing a trailer and a car at the southern end of the Terrace Tunnel at 7.50am.

UPDATE 8:55AM
Due to this crash, the Terrace Tunnel remains CLOSED in both directions. Next update by 11am.

Please avoid the area and delay travel as congestion is heavy throughout the city and motorway. Serious Crash Unit are attending. ^IF https://t.co/tRCZ6PY4kb pic.twitter.com/vOO5HYmb8e

— NZTA Wellington (@NZTAWgtn) August 28, 2018

A witness to the aftermath of the crash said she was walking to work over the Terrace Tunnel when she saw emergency service vehicles.

"I was walking above the Terrace Tunnel on Macdonald Crescent so looking down I saw all the emergency services that were there.

"There was what looked to be a Crown moving van with a trailer and a silver wreck which was completely totalled," she said.

"The ambulance staff were working on someone extensively. I was thankful I couldn't see great detail of who or what they were working on but there were people on the overpass craning over."

The witness said it appeared a northbound vehicle may have clipped a vehicle in the southbound lane.

"I know the speed at that time of day is pretty slow moving going south, but the northbound lane tends to be able to go faster in the morning," she said.

"It was surprising to see that in central Wellington because you don't usually have speeds where you would have that kind of car damage."

The Serious Crash Unit will be attending and the area will be closed for some time, police say.

Motorists are advised to avoid the area and take alternative routes wherever possible.