Dick,
Of course you don't have to fly the extinguisher to be checked: drop it off when you're next in town! Have you checked
AD Gen 65? As a private <2750kg helicopter you are not required to have an extinguisher anyway.
If you carry an extinguisher it should be AS1851:2005 compliant (for AS/NZS 1841 hand held fire extinguishers) which mandates a 6 month, 12 month and 5 yearly recurring inspection, hence the call for a 6 month re weigh.
All this is regardless of aircraft type: your A109E should be the same as well. I'm very surprised this hasn't arisen before in all the years of operation, was it only just noticed?