Police have a new weapon in their arsenal: large gas-powered sponge bullets.

The new tactical option was made available to Auckland, Wellington, and Christchurch police as part of a pilot programme in 2013.

After an endorsement from the Independent Police Conduct Authority, the weapons will now be rolled out to the remaining Armed Offenders Squad units.

Superintendent Chris Scahill said the weapon was used during the arrest of Aaron McDonald, who fled a murder in Christchurch before attacking two tourists on the West Coast.

An IPCA investigation was launched following McDonald's arrest. The findings were released this morning, endorsing the police use of the sponge round "when they were faced with apprehending an aggressive and threatening man".

"The first of two sponge rounds was fired at McDonald's legs after CS gas was used to force him out of his car, causing him to stumble. A second round was then fired, distracting him, at the same as a police dog was released, which engaged Mr McDonald and took him to the ground, enabling officers to restrain him," the IPCA report said.

IPCA chairman Judge Sir David Carruthers said although McDonald did not sustain any serious injuries during the incident the Authority was asked by police to undertake an independent investigation given it was the first time the sponge round had been deployed by officers.

"The Authority found that the police involved responded appropriately to this situation and that their command and control of the incident was excellent. The Authority also found that the level of force used to apprehend McDonald was justified in the circumstances," said Judge Carruthers.

"While the sponge rounds did not incapacitate Mr McDonald, they were effective in distracting him, and their use in conjunction with the Police Dog Team was effective. Police are now planning to make the sponge round available to all Armed Offender Squad groups throughout the country, which the Authority supports."

The round will now be made available to the remaining 14 AOS units around the country, who will receive appropriate training in their use.

Superintendent Scahill maintains the weapons will only be used sparingly.

"That said, use of the sponge round is still likely to be relatively limited, given that it is only available to specialist groups, and will not be viable to use in every situation."

The 40 millimetre projectile is designed to spread its energy across the surface of its target, meaning it's still non-lethal even at extreme close range.