I don't know how much you already know about currency effects, so I'll start from scratch in describing what I know.
There are a bunch of things that affect the value of a currency, and thus exchange rates. These include productivity, inflation, interest rates, and the popularity of the currency as an international standard of exchange, as well as short term anticipation of these effects by currency traders.
The productivity of the home country affects exchange rates because it affects how much the country produces, which affects how much a given amount of that currency can buy. Inflation affects exchange rates for the same reason, but in the opposite direction. These taken together are the biggest long term effect for stable countries, because they affect the real value of the currency (how much tangible stuff you can buy with it).
Interest rates affect currency values because they affect the financial returns of the banks and traders who hold a lot of currency - most of their holdings are typically in the form of interest bearing investments rather than just in cash. In the long term - over decades - interest rates tend to compensate for the effects of inflation, since interest rates follow inflation in the long term. In the shorter term, measured in years, they tend to move in opposite directions, since inflation rates are negatively affected by short term interest rates. Currency traders make their money by anticipating these effects, favoring currencies with high interest rates relative to inflation, since that allows them higher returns without much inflation erosion of those returns.
The popularity of the currency as an international standard of exchange is a little different. Basically, if a currency is popular for large international transactions, there will be demand for it just as a means of conducting these transactions, without much regard for the economic behavior of the currency. That will result in extra demand for the currency, which will
boost the value of the currency relative to economic fundamentals.
The U.S. dollar has fallen sharply over the last couple of years because the Federal Reserve has been trying hard to avoid deflation over those years. This means they have been holding interest rates low - bad for currency traders - in order to avoid deflation - also bad for currency traders, as deflation is good for the value of the currency, even though it would be very bad for the economy as a whole. I'd guess this effect probably accounts for about half of the 50% change you've seen in the cost of that equipment in dollar terms. I expect this effect to reverse in the next year or two, as the deflationary threat has receded and the Federal Reserve has shown every sign of being willing to raise short term rates as a result.
In my opinion, the main open question regarding the dollar over the next few years is whether it will remain the international exchange currency of choice, and in particular whether OPEC countries continue to favor it for oil transactions. In the past, the dollar has achieved this position by being the premier currency from a large nation with both political and economic stability and strength. The euro is now competitive with the dollar in terms of the size and strength of its economic base, and probably in terms of the political stability of that base as well, so we might see some shift away from the dollar and towards the euro. This would reduce the value of the dollar somewhat, though in my opinion it's not likely to have a bigger effect than the reversal of the deflation fighting effect I noted in the previous paragraph.
I'm not very familiar with the New Zealand economy, but it's possible that a few years ago their currency was undervalued, perhaps for reasons of inflation that have since been fixed.
Have you looked into the issues of allowing the lumber to season properly to avoid warping after installation?