IPCA chairman Judge Sir David Carruthers said police had complied with the law and police policy in both cases.
"Both incidents were caused by the actions of the drivers and subsequently ended in tragedy," he said.
The first pursuit started after Mr Stehlin, an unlicensed driver who had been prohibited from driving, failed to stop for a red light and sped along Great South Rd in Papatoetoe.
The pursuit continued along State Highway 20, where he momentarily lost control of his car and struck the left-hand barrier.
Mr Stehlin regained control of the car and continued driving at speeds of up to 160km/h, but police abandoned the pursuit in accordance with police policy.
A second pursuit started after Mr Stehlin overtook a patrol car at the Massey Rd off-ramp. The pursuit was abandoned after he showed no signs of stoping.
The car lost control at a right-hand bend 500 metres further down the road. The car struck the kerb and slammed into a parked Toyota Hilux on the other side of the road.
Mr Stehlin and Mr To'o To'o were killed instantly. The other two passengers died later in hospital.
The IPCA found police complied with the law and police policy in starting the pursuits and with regards to communication, manner of driving, on-going risk assessment and abandonment of the pursuits.
"This was a tragic accident caused by the actions of Mr Stehlin, who was driving dangerously and in excess of the blood alcohol limit for a person of his age," Sir David said.
"These actions resulted in the loss of life of four young people."
The IPCA said the second incident happened after Ms Nelson-Boyd failed an alcohol test at a checkpoint in Wanganui and had her keys confiscated by police.
Later that night, she got a spare set of keys to her car and drove six friends to a party in Bulls.
When she returned to Wanganui, officers in a patrol car suspected her car was overloaded because of the angle of its headlights.
Ms Nelson-Boyd failed to stop when police activated their warning lights, and continued to evade police for 3.7km.
When the patrol car turned into Swiss Ave, the officers saw her car had collided with a tree.
Officers immediately notified the police dispatcher and requested an ambulance before helping the car's occupants.
Sir David said the pursuit lasted just over two minutes in a residential, well-lit area of Wanganui.
"Police complied with the law and police policy in commencing the pursuit and regularly assessed the level of risk throughout the pursuit," he said.
"While in these instances the authority has not found any breaches of police policy, the authority is continuing its discussions with police about a review of policies connected with the pursuit of fleeing drivers."
