The 16710 is a very nice watch, it was the first in the Rolex GMT series that has the settable hour hand. Very nice if you are traveling, as you can leave the GMT hand as your home time and reset the hour hand for your destination time zone. The disadvantage of the 16710 movement is the quickset date feature was lost when they went to the new movement in the 16710. That may not be a big deal to you if you wear the watch all the time, but it is if you only wear it occasionally.
It looks like 6.5k NZD converts to around 5220 USD,so your dealer is right in the ballpark. I looked at a couple of reputable sites, and it looks like the prices are running from around 4800-5500 USD for 16710's in very good to excellent condition some with recent services, all had boxes papers, etc.
Condition is evertything, recent service history is important unless the dealer is willing to include a service with the watch sale. Boxes and papers nice, not totally essential, but will make it easier to resell later if you should change your mind.The nice thing about buying a preowned one like this, pretty much all the depreciation has been taken, so you could wear it for a few years, and get most or all of your money back if it's still in nice shape.
I have a 16750 which is the model right before the 16710. It's absolutely one of my most favorite watches.
I went back and looked at the watch on the website link. that one looks to be in mint condition with box, papers, etc. I would think that this is a pretty good deal. also it looks like the dealer handles quite a few high end watches, so they aren't some shyster operation.If iwere in the market for a 16710 and this one came up close by, i would certainly give it a go. Any room for negotiation with these folks, or they really firm on the price? Lots of times if you walk in with cash in hand, dealers will negotiate. Tell them, you really like the watch, but you have 6k to spend., can't hurt to try!
Good luck