We probably would only have 10 days or so after travel time to spend.

Would it be reasonable to just devote our time to the South Island instead of both North and South in August ?

Would this allow a leisurely loop drive/trip from Christchurch to Queenstown and back ? Is this too much time in August ?

We hope to do some hiking and maybe ski. Though the North Island might be milder, really cannot resist going to the South.

A 10-day loop around the S Island is a good amount of time to not feel like you are rushing about. The weather will not be ideal at times, particularly on the west coast and at higher altitudes, so it would be a good idea not to develop a tight and rigid itinerary. For example, you may want to establish a base in/near Fiordland and wait for a weather window to take a Doubtful/Milford Sound cruise or a walk on Franz Josef/Fox Glacier.

You also might want to consider heading to the North end of the South Island, which should have somewhat better weather. It might be moderate enough to kayak/hike along the coast of Abel Tasman Park.

Although the North Island will have better weather in August, if you are going to the South Island, I would stay on the South because you don't have enough time to do both very well.

Thanks four suggestion ALF.

Is Queenstown a good base for the majority of our stay, ie to access Milford or Fox Glacier as you suggest, or is it too far from those sites ?

Also I understand it will be ski season, which might mean Queesntown will be expensive.

Can you suggest some possible bases- perhaps two or three at the most for the duration of the trip ?

No Qweenstown is not a good base for everything. Too far. It's too far to drive back and forth to the glaciers... think of staying in Frans Josef.

Heading the other way, it is possible to see Milford from Queenstown, but I'd stay closer.
I'd suggest you see, move on, see, move on... etc

Would it be reasonable to go CHC-FJ , FJ-Queenstown, Queenstown-Dunedin, Dunedin-CHC and have several days at FJ, Queenstown, and Dunedin each ?

Is there enough to do around FJ and Dunedin for 2-3 nights, maybe longer at Queenstown ?

Where is the best base for Milford in August ? Thank you.

That is a good itinerary. We stayed in FJ for two nights and Dunedin for three and had plenty to do! IMO the best base for Milford is Te Anau. I would spend 3 nights in Queenstown and 1-2 in Te Anau if you have time.

2 nights CHC
2 nights FJ
2-3 nights Queenstown
1-2 nights TA
2 nights Dunedin

thank you for your suggestions. Good to know about Te Anau as base for Milford.