Yes, they should be getting checked, after five years, they are due for a pressure test (for small ones, this is usually uneconomic and they are replaced). If they are mounted outside of the vehicles cab, six monthly would be more appropriate

We have in place the annual checks so that we can see if they are serviceable and in some cases have not been fired off, above that, we also get drivers to take them out once a month as part of their checks and to shake them. This stops the dry powder from settling and getting hard at the bottom of the extinguisher. And as Steve has already pointed out if they are mounted outside the vehicle 6-month checks would be more appropriate.

Thanks guys I did find information under NZS 4503:2005 for hand operated Fire Equipment. "It is a requirement that all portable and wheeled fire extinguishers be inspected at least every 12 months" and there are no set standards for the inspection of fire extinguishers for cars. (Light vehicles).
This is conflicting information to what I was told by the company that completes the annual checks on the office fire extinguishers. He stated that all the work vehicles, over 20 of them require the same annual testing and tagging at the same cost as the office ones. I think we will adopt or own regular checks as suggested and replace any that appear substandard and conduct the shake test.

Yes they do tell you that but at the end of the day they are there to make money.

and there are no set standards for the inspection of fire extinguishers for cars. (Light vehicles). — Chrissy Hansen

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NZS 4503:2005 for hand operated Fire Equipment — Chrissy Hansen

Annually unless in an arduous location as per NZS4503:2005, the fact that the portable fire fighting equipment is mounted in a vehicle is neither here nor there. What's not required, is putting a fire extinguisher in the vehicle in the first place.

A 1Kg Fire Extinguisher has no place anywhere, if you are going to supply a Fire Extinguisher, put a decent one in that has 1/2 a chance of putting a fire out 2.5Kg minimum, with a flexible wand to direct the contents of it at the appropriate part of the fire according to what is actually burning.

Under NZS4503, you are supposed to supply training also (and as per Heath & Safety At Work Act)

4503 is up for review soon as I understand it.
Those dry powder extinguishers in vehicles need to be shaken monthly, as has been said previously. The vibration of a vehicle will cause the powder to settle quicker than in a building.

To be honest, there isn't much to check: condition, give it a shake, and that the pressure gauge is still in the green. And make sure it doesn't rattle when you shake it - one of the failure modes is the internal standpipe failing.
This is exactly what your extinguisher servicing people are doing, so they are telling the truth when they say that they need the same checks done.

This is exactly what your extinguisher servicing people are doing, so they are telling the truth when they say that they need the same checks done. — Aaron Marshall

• They should be checking when the extinguisher is due for a pressure test (Five years
  after it was manufactured).
  
• They will confirm the instruction for use label is present and can be easily read
  
• They will check the pressure gauge is in the green zone (over/under is a fail)
  
• They should be weighing it to confirm it is still at it's initial fill weight,
  
• They should be removing any flexible wand and confirming it's not blocked (wasps use outdoor
  ones to try and start a nest off in), and any internal rattle
  
• They are examining the outside for any signs of corrosion/damage(they are pressure vessels)
  
• A gentle tap with a rubber mallet will free up any compaction (you should feel the powder
  sloshing around in side it- the center of its gravity will change too if it's not compacted.
  
• They will confirm that locking ring is fitted, and indicator tag is in place.
  
• Once they have done that, they will clip the test date tag on the extinguisher with the month
  and year of this check.
  
• A good operator will give you a list of all the extinguishers they checked, what their weights
  were, and when their next pressure tests are due

No reason why you can't do this, but you must do all of this

most of that comes under what I would call "checking for condition"

The other thing to consider is that the Standard calls for checks to be carried out by 'a competent person'.
Whether you consider someone competent will be up to you (and the Judge, if it all goes wrong).

I just brought this up with someone who helped write the previous version, and is looking at revising it now, and this is one thing that they've flagged as needing attention. However, qualified and competent are different things, so wording this is going to take some work.

And, this maintenance is all based on dry powder extinguishers, CO2 or other forms are different.

However, qualified and competent are different things, so wording this is going to take some work. — Aaron Marshall

And frequently in my experience, "Qualified" persons aren't necessarily competent to undertake some work. But we are talking a Level Two check of a Fire Extinguisher, not a Pressure Test/Recharge, and as such, isn't maintenance with the exception of removing the Wand/Hose from an FX so equipped and checking it isn't blocked.

Some times a picture does paint a thousand words