She was described as an Asian female who was in a grey car parked near the crash scene. She apparently spoke to another witness about the trike and expressed her concerns about how the trike had been driven.
Mr Brown said the woman was possibly parked on Carnegie Rd, opposite Ford Rd.
"We are very keen to speak to her as she may be an important witness that could help us clarify events leading up to the crash.
"We would stress that this person was in no way involved in the crash, but may have seen something in the lead-up to it."
The police spokeswoman said investigation into the crash was ongoing.
Police were unwilling to say if speed, alcohol, or driving behaviour were factors in the crash until the investigation was complete.
The crash was the second involving a trike in Napier in the past few years.
In 2011, a man was test-driving a 1200cc trike in Greenmeadows, Napier, when it veered off a slightly curving stretch of road, mounted the footpath and crashing through a power box and metal fence, killing him.