He managed to slow down to about 40-50km/h.
But he later told police he didn't want to make an abrupt stop, because it would've been "uncomfortable" for his passengers.
The horse float, which had been travelling at 80km/h, braked heavily but was unable to avoid the collision.
Mr Chew's side took the brunt of the impact and he had to be cut free of the wreck.
He suffered bleeding and swelling to the brain as well as significant internal bleeding.
Steward Vanessa Leonara Savio Coelho, seated in the middle of the back seat, also wasn't wearing a seatbelt.
She had to be cut free by rescuers and underwent surgery the following day for injuries that included a fractured arm and shattered pelvis as well as spleen and bladder injuries.
Two other passengers were unhurt.
Wu, of Singapore, pleaded guilty to two reckless driving causing injury charges last Friday.
Today at Christchurch District Court, defence counsel Kerry Cook described the incident as a "tragic unintended accident with tragic consequences".
In what Mr Cook said was "a cruel twist", Wu wasn't meant to drive that day but was the only one up when the rental car arrived.
He was driving on an unfamiliar road, taking directions with others from the GPS system, and had not been speeding, Mr Cook said.
Wu saw the stop sign "very late" and "made a split second decision that was to have unfortunate long-term ramifications".
The injuries to those not wearing seatbelts were "horrendous", Mr Cook accepted.
Judge Stephen O'Driscoll noted that it was "perhaps ironic" that there was not injury to those wearing seatbelts.
He said that it would have been "prudent" for Wu as the driver to ensure his passengers were all wearing seatbelts.
But as Mr Cook outlined, there is no legal responsibility on the driver of a car to ensure passengers aged 15 and over to wear seatbelts.
Mr Wu, largely uninjured, tried to help his workmates at the scene and immediately acknowledged his wrongdoing.
He is genuinely remorseful, Mr Cook said, and has apologised face-to-face to Ms Coelho and Mr Chew's wife who "don't hold any grudges against him".
"He is significantly upset and distraught at the harm. He has a significant burden to carry," Mr Cook said.
Before today, Wu had already paid into the court trust account $15,000 for emotional harm reparation, with Mr Cook saying that although he knew it wouldn't fix all the harm he had caused, he hoped it would help in some small way.
It was fortunate there were no fatalities, said Judge O'Driscoll.
There had initially been "real concerns" as to whether Mr Chew would live, he said.
The judge had now seen a medical report which indicated he has suffered serious injuries that included a mild head injury.
He had 6-12 months of rehabilitation ahead of him, the court heard.
A victim impact statement from Mr Chew's wife said he had undergone surgery and would require further surgery, with his recovery in the future still being "unknown".
"It is obvious that he will face a very long recovery," Judge O'Driscoll said.
Ms Coelho would "bear scars for a long time" and was unsure over the future of her job, the judge said.
In a statement released to APNZ after court today, Wu took responsibility for the tragic accident.
"I sincerely apologise for all that has happened," he said.
"It was never my intention for any of this to happen.
"I take responsibility as the driver of the car and I am truly sorry for what has happened.
"I am thankful that everyone involved in this accident is showing signs of improvement and recovery and I just wish we can move on from here so my friends and colleagues can focus on becoming healthy and well again."
