If you are worried you could always send IRD the GST on the $12 (=$1.80) in the post each month, in cash (which is legal tender), with a letter stating that you are doing the right thing by not dodging GST and paying what you owe, and ask for a receipt each time.

Yes I'm being silly, but only slightly. Whether they bank the money (which will probably cost them $50+ in admin each time) or tell you to stop, you will then have cast iron proof that you tried to pay the GST. Ergo, if they try to ping people for tax avoidance later, you can say you were doing it for geoblocking and not tax avoidance and the produce proof you tried to pay. Which should be enough for the judge.

Plus it would be a useful way of getting rid of those 20 cent and 10 cent coins that tend to accumulate in pockets over time.

(Plus, if done on a large scale, it would also be amusing to see how IRD tried to deal with tens of thousands of cash payments of less than $2 a month, all of which had to be recorded, banked, accounted for and receipted )