A police officer's incorrect use of road spikes caused a pregnant woman to be hurt when her car was rammed by a vehicle driven by two teenagers, an independent report has found.

The two teens had been driving a stolen Mazda in South Auckland on February 28 last year, when police began a pursuit at 1.52pm in the suburb of Manurewa.

The pursuit continued through Auckland's motorways and was headed north of the city on State Highway 1 towards the Johnstone Hill tunnels near Puhoi.

However, as the pursuit approached, a police officer mistakenly threw road spikes across the road in front of a car driven by a pregnant woman, a report by the Independent Police Conduct Authority found.

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This caused the pregnant woman to hit her brakes and the stolen Mazda to slam into her vehicle from behind.

The injured woman was taken to hospital, but has since given birth to a healthy baby.

The authority found "that, in principle, it was appropriate for road spikes to be used to stop the fleeing driver".

However, the officer had not chosen the right location to lay the spikes or adequately assessed the risks, it said.

"In his haste to deploy road spikes to stop the fleeing driver, the officer did not consider the basic principles of police policy and prioritise the safety of other road users and himself," authority chairman Judge Colin Doherty said.

"As a result, an innocent member of the public suffered injury and trauma."

The officer had been keen to lay the road spikes because he was becomingly increasingly concerned - while listening to police radio - that the pursuit was approaching a dangerous section of road between Puhoi and Warkworth.

Realising, he was the only officer ahead of the pursuit who could prevent the fleeing driver reaching this stretch of road, he was given permission at about 2.30pm to lay road spikes.

However, the authority found the location he chose did not comply with police policy because "it did not provide sufficient cover so as to ensure his safety and that of other road users".

He also wasn't wearing high visibility clothing or an easily identifiable police uniform and had not activated his car's emergency lights.

Then when the pregnant woman and the Mazda approached, he misjudged the speed of the vehicles, the authority said.