It is not about taxes. It is about a country's GDP (gross domestic product(s))export to import ratio. The more a country imports and the less they export determines the value of their currency and foreign exchange rates. Meaning if you import a hell of a lot more than you export your currency will devalue and provide less buying power hence the reason for the high cost of any import. If export prices are low conversely import prices will be sky high which has long plagued New Zealand.
Also factored in are how many units any given import will sell in any area which will drive the price down simply through volume. New Zealand and Australia are completely surrounded by thousands of miles of water meaning a finite number of units that can sell equating to still higher prices.