The owner of the trailer the cyclist fell under, who asked not to be named, was the backseat passenger in a Nissan Navara towing it.
The collision happened at the intersection of Quay St and The Strand the morning after the Rugby World Cup opened.
"All I saw was the cyclist fall and then my husband looked in his mirrors, because we were in the middle lane ... He looked and saw the cyclist go under our trailer. And that's when we stopped."
She said the incident was over in a couple of seconds. "We didn't hit her, we were just driving past. I said, 'That poor bugger has fallen off his bike', and he looked in his mirrors and said, 'S***, he's gone under the trailer'."
They didn't feel anything because the trailer was so heavy.
Her husband and brother went to see if they could help others who had gathered around where the cyclist, who they learned was a woman, had fallen to the ground.
She told the Herald it must be upsetting for the woman's family. "There's not a day that goes by that we don't think of her. We keep in touch with police and they fill us in how she's going - it's really, really sad."
They had told police all they knew. "I don't know what made her fall, whether someone knocked her. I know we didn't. All I know is she fell and unfortunately she fell under our trailer and has gone under the wheels."
Constable Mark Rodgers said it was believed the cyclist had "made contact" with a vehicle at the intersection and then was run over by the trailer.
"The inquiries we have made have identified vehicles that were behind the crash scene or approached the intersection just after."
The dark BMW convertible was right behind the point of impact and would have had a clear view of what happened.
"They would have seen the crash and be able to give us some detailed information," said Mr Rodgers.
He hoped to be able to tell the cyclist's family exactly what happened. "But at the moment we're not 100 per cent sure."
It wouldn't help the cyclist, who hasn't regained consciousness since the accident. "But if we can find out what happened, hopefully [we can] prevent it happening again."
One of the cyclist's daughters, who is aged in her 20s, had flown from the United States to be with her mother. Another daughter is unable to come to New Zealand.
The cyclist has been living in New Zealand for a number of years.
"We know she made contact with the driver's side of the vehicle to her left as she passed through the intersection," said Mr Rodgers. "She has then fallen ... and the trailer to her right has then gone over her head and caused quite serious injuries."
Unless someone came forward with fresh information it didn't seem like the drivers of the two cars the cyclist had come into contact with would face charges.
"Unless we get independent witnesses to say different, it appears the two drivers involved may not be responsible for the crash as such. It may be the cyclist has lost her balance, collided with one of the vehicles, fallen from the bike and unfortunately been run over by the trailer."
Mr Rodgers said investigators hadn't found anyone who could tell them what the cyclist was doing before the accident or where she was heading.
She hadn't been wearing a helmet but given the weight of the trailer, it was unlikely that would have made any difference, he said.
Mr Rodgers urged anyone who knew the driver of the convertible to tell them they needed to speak with police.
"They are potential witnesses, not a suspect."
CAN YOU HELP?
Anyone with any information should call Mark Rodgers on (09) 359-3116 or email mark.rodgers@police.govt.nz
