Reputable Honda Motorbike Dealership in Da Nang?

Stick with the ones that are "Honda Head" with the red and white decor.  You can search "honda head da nang" in Google maps.  I assume the name derives from the English meaning of head, but these are the company dealerships.  Some may give better service than others but you can sort that out by trial and error or advice from locals, after you buy.  All will honor warranty and service contracts. 

Other dealerships  may be licensed to sell new Hondas but warranty and service will be not as good.  Over my objections but because my wife wanted a certain color that was not available at the Head, we bought an Air Blade from a third party dealer.  We got only a one year warranty and one year free servicing instead of two which she admitted later was a mistake.

Shipping back to AU, the obstacle might be air pollution testing if the PCX is not sold there.  I do see that Honda in the US seems to sell what looks like the PCX with another name.

Honda takes pride in customer service so stick to Honda main dealers, not the tiny dealers you see dotted around all over Asia.
Honda also has excellent service facilities so always use them at the required times in order to keep your engine warranty valid.
The bike you're considering is slightly larger than their other scooters but still fine either in crowded cities or country road driving. You'll find having a footwell is a massive advantage in and after a rainstorm.

Before you buy, check out Yamaha as they have a lot of very nice bikes and some have larger underseat storage that will take waterproofs as well as a helmet.

http://yamaha-motor.com.vn/

One more. A model with LED headlights is probably a good idea.
Apart from lasting a lot longer, the lights are far brighter so light up the road better at night.

Last note - What advantages (apart from looks) does the larger bike have over the slightly smaller scooters?
If you have trouble answering, save a lot of money and buy a more standard style.

THIGV & Fred- Thanks for your speedy replies.  I will definitely stay with the main Honda dealers

Just looked at some discussion on bike import to Aus and common thread seemed to be for cheaper more common bikes, the import process is quite involved and not worth the hassle.

Fred - have gone with the PCX as it is a little larger (I am 6'3") and my wife will be on the back and I understand it will be more comfortable for her.

Cheers, Rob

I agree. The bigger bike is a good idea for two people, especially when one is larger.