In the lead up to a situation where officers on the front line have to make a decision to pull the trigger and use deadly force, police have to follow certain procedures when using firearms.
The victim of Wednesday's shooting is just the 21st person killed by police since 1941.
Murray Forbes, who led the armed offenders' squad into Aramoana, says the decision to pull the trigger would have been a difficult one.
"His mind would be flat out, he'd be pretty busy trying to think of these things, trying to think of fire orders, trying to think of what the fella has got, trying to think about the people behind him, if you draw a firearm who's behind you what if you shoot someone else - he's got a thousand things going through his mind," says Murray Forbes, former police officer.
The New Zealand police force of around 8000 officers has an arsenal of nearly 1,800 pistols and 1,300 semi automatic rifles.
Unless it is an emergency, only commissioned officers can issue firearms to staff and police procedures are strict on their use.
Police can only open fire to defend themselves if they fear for their lives or those of others.
They can also use firearms to make an arrest or prevent an escape if an offender poses a serious and harmful threat.
But before an offender can be shot, they must be warned and called on to surrender and officers must make sure they cannot be disarmed or arrested without being shot.
The fatal shooting of Waitara man Steven Wallace in April 2000 also prompted questions about the use of firearms. Senior Constable Keith Abbot was cleared of murder after a rare private prosecution by the Wallace family.
A year long trial of Tasers in four police districts ended last month and there are now calls for them to be introduced immediately.
New Zealand First MP Ron Mark says if Tasers had been available on Wednesday night it is likely the officers involved would not have resorted to using lethal force.
A report on the effectiveness of Tasers is due out in December.
