He did not activate his siren as he followed the cars down Pioneer Highway near Longburn at high speeds.
Transcripts given to the IPCA show he told a communications officer he was not pursuing the cars, despite travelling at high speed in a 50km/hr zone.
The chase ended when Downes braked suddenly to avoid another car.
His Honda slid across the road and hit a tree before landing in a drainage ditch filled with water in two pieces.
Downes died at the scene.
The pursuing officer did not see the crash and went on to arrest a 16-year-old pair who had been travelling in the other car when they crashed at a traffic island.
IPCA findings show the officer should not have followed the two Honda cars at speed without his warning lights activated.
Telling communications officers he was not involved in a pursuit when he was travelling at high speed and following an offender broke police policy, the authority said.
In a second pursuit at Lower Hutt, an officer pursued 26-year-old Benjamin Eden after seeing him accelerate sharply out of a roundabout.
The officer suspected Eden was under the influence of alcohol, though toxicology results later found that was not the case.
Eden, who had a history of driving offences, drove away from the officer at speeds of about 150km/hr in a 80km/hr zone.
He crashed into a power pole while trying to navigate a bend and received serious head injuries.
The IPCA found the officer failed to activate his warning lights during the pursuit.
It said matching Eden's speed of 140-150km/hr was also "undesirable" in the circumstances.
Both officers were referred for remedial training. The IPCA recommended police clarify their police around following vehicles at speed.