To become a member officers must first complete their two-year probationary period, then they are able to choose a career pathway within the police. The AOS is a part-time role so is in addition to an officer's career development.
The first step is a pre-selection course held locally to check that the officer is a good fit to the AOS.
"It's so the person is aware of what they can expect of the national selection," Mr Grimes said.
"If they get through that then you're able to go on the national selection course, which is three days and that's pretty intensive, not just physically but also mentally."
Then there is the national qualification course, which is a further four weeks.
They don't just learn how to fire a gun. There is a lot of work on decision making, being able to work in stressful conditions, being physically fit as the gear weighs at least 15kg and other skills.
"Joining the AOS appealed to me because it is a special squad. I had limited knowledge about how they operate, they were recognised as being a professional, well-organised group within the police. What attracted me also was the sort of team environment that AOS is all about.
"There's no place for an individual in the squad.
" You have to be good at operating under stressful conditions, the ability to make sound decisions and to operate for long periods in unfriendly environments."
Resilience and comradeship are two other words he mentions.
Its members are on call 24/7 unless they have grounds not to be available. This can mean being called out in the middle of a normal police shift to being deployed in the middle of the night or as soon as they've got home from work.
They get called out to all manner of jobs - anything that is marked as high risk and where weapons and firearms are involved. They also attend high risk search warrant exercises.
It's a large area they cover - Putaruru, Tokoroa, Taupo, Turangi, as well as Rotorua.
Rotorua has a proud track record, since they were formed in 1967 no shots have ever been fired by its members in the line of duty. Instead they use other tactics, initially cordon and contain and then negotiation or other options available to regular police officers, such as pepper spray or handcuffs.
Mr Grimes has been involved in callouts which led to officers being killed. In 1999 Murray Stretch was killed in Mangakino and in 2009 Len Snee was killed in Napier. The Rotorua AOS was called out to both.
"One was more tense than the other but in both the offender was still active. There are other jobs you remember and talk about after but these are the two that stick in the mind the most. We know we're there to do a job and ultimately it's about catching whoever is responsible to ensure the rest of the community and other police officers are going to be safe.
"That's why we do it. By catching them we're making sure that everyone else is safe."
Past and present Rotorua AOS members will mark 50 years of armed offenders squads in New Zealand next Saturday.
