GST - the itch to impose differential rates of tax

The common taters who itch to Fiddle with GST need a reality check. Changing the overall rate is something of a mission, but do-able: the change from 12.5 to 15% shows that.

But the core feature of GST is that it has, effectively, only two rates: zero (for financial services and the like) or 15% for everything else.

[Yes, before y'all point to them, there are tiny exceptions, like the reduced rate for long-stay residential care in managed village facilities, but these are well able to be coped with where encountered because those organisations are typically single-purpose anyway].

The moment GST becomes differential for any reason, in any widespread sense (e.g. for food versus petrol) the complications start rolling, as do the domino effects:

1.     Every business exposed to mixed sales which cross the new GST application boundaries, have instant compliance costs. Sales have to be differentiated, GST returns become much more complex (because the possibility of getting it wrong multiplies, and IRD takes a dim view of errors). These costs translate into smaller profits, which lead to price increase pressures, business withdrawal or downsizing, and other direct costs.
2.     Differential GST requires exquisite, certain Description of every possible good or service to which a given rate applies. Supermarkets typically run 20-40K SKU's, and each and every one must be Described accurately and the correct tax rate applied. Maintenance alone is a big overhead, and the effect on typical sales techniques such as 'discount bundles' - buy three 2-litre milks, get 1 free and get a half-price pair of gardening gloves - can be easily imagined. How should this be taxed, because the weighting of the sales is uncertain.
3.     The BOM-explosion issue arises very quickly. I buy Flour, Salt, Yeast and Butter. Are they 'Food'? Not individually but together, they make Bread. How to tax them? Go one step further back. Pepper, bay leaves, cinnamon. Food? Less certain. But a hot-pot meal would be bereft without them. Go a further step back in the BOM. Pepper grinder, pestle and mortar. Food? No, but try making that hot-pot without them. And they cannot be used for much else. Such boundary issues destroy the Certainty which is the hallmark of a good tax system.
4.     The central administration needed to maintain the endless lists of goods, disseminate it and keep up with the constant flow of new product (whoever heard of quinoa a few years back?) is non-trivial. This is pure economic deadweight: it is substantial extra activity in a non-tradeable.
5.     Once differentiation has commenced, the political arguments about stuffing up a nice simple tax, go out the window. The technical term is Defenestration. So every new Bright Idea has a much better chance of being incorporated (because 'It's Good for the Children/Environment/Winnie/our little business segment/our Region/our political cronies/the unions/beneficiaries/the Grey Vote...it's an infinite list). Do y'all really wanna open That up for grabs? It's a direct path to cronyism and to a tax guide the size of several phone books, updated quarterly, available for purchase for a Modest Fee.

There are two aphorisms worth recording about Tax in general.
- it's like plucking a goose. You want the most Feathers with the least Hissing.
- a high degree of Inequity is preferable to a small degree of Uncertainty.

The short version: don't foobar GST by opening the Pandora's Box of differential rates.....