A new projectile dubbed a 'sponge round' has been approved for the New Zealand Police Force.

The Independent Police Conduct Authority has approved the new bullets, which are fired from a 40mm gas launcher.

The new foam bullets are supposed to incapacitate offenders instead of causing serious lifelong harm.

The weapon was introduced to Auckland, Wellington and Christchurch armed offender squads in November 2013.

The foam bullet is supposed to incapacitate an aggressive person, causing bruising rather than significant or long-lasting injury.

The option has now been rolled out to the remaining armed offenders squads around the country.

"The sponge round provides these specialist police staff with an effective tool that can help resolve dangerous incidents from a safe distance, where previously firearms may have been the only remaining option," says Police national manager response and operations superintendent Chris Scahill.

However, use of the device has been linked to the death of a prison inmate in California. He died after a guard shot him in the head with a sponge round.

Chris says any use of force by police carried an element of risk.

"The conditions under which any tactical option may be used will always be dictated by the actions and behaviour of those we are confronted with at the time.”

The device has only been used twice in the apprehension of Aaron McDonald in March 2014 after he had murdered a woman in Christchurch and attacked two hitchikers.

Independent Police Conduct Authority chair Sir David Carruthers says the police responded appropriately.

"The Authority also found the level of force used to apprehend Mr McDonald was justified in the circumstances."

Sit David says although McDonald did not sustain any serious injuries during the incident the authority was asked to undertake an independent investigation given it was the first time the sponge round had been used by police in this country.

"Police are now planning to make the sponge round available to all Armed Offender Squad groups throughout the country, which the authority supports."

The results of the investigation have been reported to the police commissioner.