New Zealand hasn't assembled a mainline locomotive in decades, and there is a good reason for that. It is the same reason New Zealand doesn't assemble cars or planes - it is far too high tech to be done in a country that has a relatively high cost of labour and hardly enough demand to justify the capital needed to do it.
It harks back to the nonsense of asking Sony, JVC and the like to disassemble TVs manufactured in Indonesia or China, so that kiwi drones could screw them back together again. China does it from scratch because the labour is cheap and demand is high - New Zealand has neither of those.
So when Trevor Mallard, keen to spend your money says "It's probably a very logical thing to do from a currency perspective, from a value for money perspective.". Well Trevor it wasn't in the 1980s when locomotives were imported complete from the UK for the main trunk electrification, it wasn't in the 1970s when locomotives were imported complete from Canada and the USA.
Locomotives have been re-engined in New Zealand, but let's face it, your local mechanic can put in a new engine in your car - but you wouldn't trust him to have get all the parts from Toyota and put it together would you?
The last locomotives assembled in New Zealand were a handful of shunters in the mid 1980s.
Now the workshops have manufactured freight wagons successfully and economically, and successfully refurbished most of the passenger rolling stock on the network. However, the new electric units Wellington will be getting in a couple of years aren't being assembled in NZ for a good reason. Rob Muldoon and the North Korean style economics of "self reliance bugger the cost" are long gone!