@nzcid I’m author of one of the Currency Switcher plugins mentioned in by fran4444, and I have quite a bit of experience related to multi-currency systems. What you are looking for can be done with some custom coding, whose complexity depends on the exact requirements.
Broad description of a possible solution
Our Currency Switcher allows to set the base currency for each product, therefore it would be possible to allow a NZ merchant to enter his prices in NZD and let the USD counterpart be calculated automatically. On the frontend, it would be possible to detect which products are being displayed, and to whom they belong, and force the active currency accordingly.
One thing to consider is that it will not be possible to show some products in USD and some in NZD. Only one currency can be active at any given time, and neither WooCommerce, nor any payment providers support a “mixed-currency order” scenario. Besides, having prices showing in multiple currencies on the same page would just confuse the customers and, most likely, result in a drop of sales.
Based on our experience, the having multiple currencies on a shop can increase conversion as long as it improves customers’ experience. If a shop becomes a mix of multi-currency prices, or if it forces customers to pay in one or the other currency based on arbitrary criteria, the effect tends to be the opposite.
