"We are looking to identify the driver of this vehicle, as it is likely they witnessed the crash."
Officers have also released a photo handed in to them by a member of the public.
In it, a number of people can be seen assisting the motorcyclist just after the crash.
Police are urging those people to come forward also, as they may be able to offer information into the circumstances of the accident.
"The people in this photo might be able to assist with our investigation and we we would like to identify them," Ms Brand said.
"It's important for us to get a clear picture of what occurred at the scene in order to determine exactly what happened and how the crash occurred."
Anyone with information can contact the Counties Manukau Police Serious Crash Unit on (09) 215 6118.