Mr Dunstan said pre-planned operations, including assisting in drug raids and executing search warrants, now accounted for almost two-thirds of callouts.
He said the squads could be called in "for other risk situations or duties where the use or possession of firearms is likely".
The risk for police staff in going to routine jobs without armed backup was highlighted in June 2010 when Christchurch man Christopher Graeme Smith shot two officers and killed police dog Gage.
The officers had visited Smith to execute a routine search warrant for cannabis.
He is now serving 14 years in prison for attempted murder and grievous bodily harm.
Last month, armed offenders squads were called to three incidents reported by the Herald, including the hunt for gunman Jamie Ginns after he shot Mata Glassie in Tokoroa.
Other high-profile callouts include the death of Halatau Naitoko, fatally shot by a squad officer who was firing at a gunman on a crime spree on Auckland's Northwestern Motorway, and the shooting of Senior Constable Len Snee in Napier by gunman Jan Molenaar.
A two-day standoff followed before Molenaar fatally shot himself.
In 2007, armed drug addict Lee Jane Mettam was shot dead by a member of the Northland armed offenders squad in Whangarei after she took a shop assistant hostage.
Mr Dunstan said the environment for squad staff had changed dramatically over the past decade.
"Offenders seem more and more willing to use firearms either in the commission of crimes or when confronted with police, necessitating greater use of AOS in support of police.
"Crime trends such as clandestine methamphetamine labs and greater appreciation by police of risk have led to increased deployments."
Mr Dunstan said another result of the changing landscape of crime was a change in training and skills.
"There is a need for new training and equipment suitable to the new environment.
"Offenders are likely to be more dangerous and prone to irrational behaviour as well as being more likely to be armed."
By the middle of this year, other frontline police officers will have better access to firearms after the Government gave permission for 2700 Glock pistols and Bushmaster rifles to be carried in locked boxes in patrol cars.
This could result in a fall in the number of armed offenders squad callouts.
The 17 squads are made up of part-time members drawn from all branches of the police.
Squad numbers range from 12 to 30 and the largest are based in Auckland, Wellington and Christchurch.
Total numbers vary but current strength is about 320 officers.
THE NUMBERS
992
Armed offender callouts in the 12 months to June 30 last year.
513
Average number of callouts in the previous 14 years.
17
Armed offenders squads in New Zealand.
320
Police officers in armed offenders squads.
