Snorch you basically have two options, (a) supply the NZMCA with the relevant form that they are requesting. It appears that they DONT have it. While you are still a financial member why not make a call to your local NZMCA S/C officer and ask if you could get a check done on your vehicle before your current membership expires? End of problem you will be all legal! There will be no cost to you if you go to the SC officer rather than him come to you, other than a thank you if you deem that suitable for his/her voluntary time.
Obviously H/O is not receiving the correct paperwork that is being requested. The office staff would not be making extra work for themselves for no reason, somewhere in all this one and one are not adding up to two. Why not put a copy of the form you have been sending H/O here on the forum and the likes of Beanpole Gordon will soon set you straight if in deed H/O should or should not be accepting your form as kosher. This all could be a basic misunderstanding of what the required form looks like.
I can envisage that you may come up with a long list of all the reasons that you have already done (a) and why you do not want to do (b) but for under an hour of your time the problem would be solved.
Or are you simply doing some wool pulling and your current SC is not quite as above board as it should be?