Try www.midistore.com, they ship to NZ and shipping seems to be cheaper relative to other US online retailers. I bought a Korg Nanopad from them.
In regard to the customs duty and GST, if your combined duty and GST comes to less than $50 NZ they waive it. This is calculated on the combined declared value of the item and the shipping amount. This will obviously in your case be in US dollars so this would be calculated using the exchange rate on the day that your speakers arrive at customs in NZ.
At midistore today you can purchase a pair of Alesis Monitor1 MkII Monitors (6.5"

for US$199. The cheapest shipping on them is USPS Priority Mail International - 6-10 days ($200.40). Total = US$399.40 = NZ$639.65
This would attract duty and GST and you bill would be 12.5% ($79.96) + 18% ($115.14) = NZ$195.10
So total cost would be NZ$834.75
Speakers are bloody heavy and shipping is done by weight. USPS is the cheapest shipping from the states. they used to have a 1 - 2 month sea freight option that was dirt cheap but that service ceased about two years ago.
I buy snowboards off ebay which is how I know this stuff.
I was looking to do the same thing from the states for monitors about 6 months ago (with the same monitors you are looking at getting funnily enough) and it is just cheaper with the exchange rates the way they are to purchase them here.
What the guy at DMC is refereeing to when he says they will go up is the fact that increases in transport costs and exchange rates will increase the retail price of future stock purchased from overseas.