The police series crash unit is investigating but it is believed no other vehicles were involved in the crash.
Witness Jenny Sutton said she was three cars back from the crash when it happened.
"The motorbike passed me and a couple of other vehicles," she said.
"Speed, risky passing, then loss of control were factors in what occurred.
"The bike skirted along the median barrier for a time before those on it were thrown."
Ms Sutton said the bike was weaving through traffic, passing very close to other vehicles but she didn't think too much of it because "that's just bikes, you know".
Then she saw a cloud of dirt and a body on the road.
Ms Sutton said she thought people were very measured in their reaction to the crash and reacted quickly, calling 111 and providing medical assistance.
"I think people managed things quite well, people were onto it. Everyone did what they were supposed to do."
Ms Sutton said her thoughts were with the families of those who died.
The crash, which occurred shortly before 1pm on Sunday, caused Wellington traffic to grind to a halt as the Terrace tunnel had to be closed, blocking the main route in and out of the capital for more than three hours.
A man and a woman died at the scene.
The Serious Crash Unit was still investigating the cause of the crash, a Wellington police spokesman said, and the full circumstances were still being evaluated.
"At the moment we are looking at manner of driving and speed," the police spokesman said.
Model Lisa Lewis, who made a well known bikini streak during a 2006 All Blacks rugby test at Waikato Stadium and is a former television topless weather presenter, paid tribute to Mr Mahon on her Facebook page.
"Rest in Peace Dee McMahon. Will always remember our photoshoot we had last year where you were a vibrant fun professional photographer with good energy reflected through the lens.
"Wishing love, strength and support to the family that have lost you..," Ms Lewis wrote.
She also posted a photo Mr McMahon took of her last year.
"Reflecting on Dee: he was a man with such energy that when you walked into a room he was in - he had such life shining bright from inside to out.
"Dee had a contagious smile and made me laugh. Dee was an Artist. He had passion and had a 'love life' about him."
