The Independent Police Conduct Authority (IPCA) has been notified of the deaths of three people in a crash during a police pursuit near Nelson early today.
Police said they would also do their own internal investigation.
Police had attempted to stop a vehicle while conducting enquiries to find a wanted person when the driver fled.
The driver crashed into another vehicle while attempting to overtake a truck, police said.
Two people in the fleeing car died along with a member of the public in another car.
Police said they were working to notify and support next of kin, as well as supporting staff involved.
"We now have three families who have lost members and our thoughts are with them during this difficult time," police said in a statement.
Police said fleeing incidents were "extremely testing".
"They are fast-moving, unpredictable and high pressure situations that require quick judgements."
Police Minister Stuart Nash said the crash was a tragedy for the families of those who died, and the officers involved.
He said police were already working with the IPCA on a review of pursuit policies and practices, and he had asked for an update on progress.
The review is due to be completed later this year.