drive from Te Anau > Mt Cook > CHC worth it?

I assume it's too late to cut Queenstown back to 4 days? (which is about right for Queenstown).
The 2 days in Te Anau is also about right, with the glow worm caves on your first evening, a day into and out of Milford, then back. (You're aware it's still 2 hours drive each way from Te Anau to Milford?)
After that, if you've got to go to Christchurch anyway, then Mt Cook makes the obvious stopping off point.
It's about an 8 hour drive from te anau to Christchurch, and yes you do have to go back through Queenstown, but you also go very close to Mt Cook - it's about a 55klm, 40 minute each way diversion off the Highway from near Twizel to Mt Cook Village.
So, get an early start, make Mt Cook by early afternoon, and then make day 2 the drive to Christchurch.

Ah, great point - I didn't realise the drive from TA to CHC would be 8 hours anyway. It definitely makes sense then to stop off at Mt Cook and squeeze in a little more adventure, rather than spend a whole day driving.

Yep, we're prepared for the 2 hour drives to and from Milford Sound but I've read that it's quite a beautiful drive and Milford is well worth the km's. I wish we had more time to do one of the Great Walks, but I've read that the day walks can be just as beautiful - any tips on which to do? Does anyone know if the Gertrude Saddle route is as dicey as it sounds?

having done the Milford day trip several times from Te Anau:

first, read this and take note:

http://www.nzta.govt.nz/projects/milfordroad/news/Transit-brochure-Milford.pdf

and note they are saying 2.5 hours one way to Milford which is realistic and safe. Rush it and die - people have.

a good walk en route is Key Summit - park at the Divide. With that, stops en route, a picnic at Milford and a cruise starting mid afternoon, you've got a full day.

no point if it is tipping it down with rain - in that case stay in Te Anau and do a sheltered walk on the Kepler track.

it also may be wet at Mt Cook - have a look from the Lake Pukaki viewpoint on the main road, and if all seems clear go back to the turnoff and go to Mt Cook. (or go straight there if you can see it is a lovely day) If not, don't bother, you will see nothing.

Yes, it takes much longer than you think to travel on these roads, google maps is unrealistic with timings.

Key Summit is a lovely walk, but as neverwinter says no point if it's raining. Alternatives are walks through the beech forests near Te Anau, either catching a boat to Brod Bay and walking back, or from the Rainbow Reach bridge back to Te Anau control gates.

I haven't done the Gertrude Saddle walk so can't help you there.

Good walks in the QT area include Lake Sylvan and to Routeburn Flats (which is a lovely spot in itself).

I recently drove from Te Anau to Milford (with a little rain) without pushing it and without stopping and did it in two hours. That included waiting five minutes at the one-way tunnel - time it wrong and you could have to wait up to 15 minutes.

I would recommend taking the Hollyford Road (unsealed) to its end and walking to the viewpoint (30 mins return) of the magnificent Humboldt Falls.

You mentioned Fox Glacier but that is on the West Coast, the other side of the mountains from Mt Cook Village. You can do a nice walk to view Tasman Glacier from Mt Cook side.

Five hours is about right to drive from Mt Cook to Christchurch.

It does take longer than you think in New Zealand. Roads are generally good, but single lane, winding and so damn scenic you can't help but keep stopping.
That said, driving non stop from Te Anau to Milford Sound, without rushing, and with a few quick photo stops, took us just under 2 hours. This was at 7am (we left early to make the 10am cruise and found ourselves with heaps of time)
We took it very leisurely on the way back, stopping at every waterfall, small track and scenic view etc, and took half a day.
Yes, the road from Te Anau to Milford is every bit as spectacular as the sound itself, but do what we did - leave early, head straight there, catch the early cruise (to avoid the lunchtime crowds and take advantage of the lighter winds in the morning). Then, take your time coming back.
There was a lovely small walk along the way to a small waterfall, but for your walks, I'd instead be looking at Glenorchy, during your stay in Queenstown. It's about 45 minutes away and has a series of very good walks of all lengths.

Thanks for your help everyone. I'll be sure to keep an eye on the weather before heading out to Key Summit or Mt Cook. Accommodation at Mt Cook is quite expensive, so if the weather is bad should we give it a miss altogether and just head straight for CHC or is there somewhere weather friendly and en route to CHC?

Stopping on the way back from Milford sounds like a good plan. Will do! Humboldt Falls sounds right up my alley as well. Also thanks for the tip on Fox Glacier being on the other side, if we do make it to Mt Cook we'll check out Tasman Glacier.

So yes - still in 2 minds about the last night on the south island, not 100% keen on spending the last night and day in CHC but if it's pouring at Mt Cook the $$ accom (plus many places are already booked out) might not be worth it either. So somewhere on the way from Mt Cook to CHC? Decisions!

From what I can remember accommodation in Mt Cook can be found for just over $100 at a lodge (Alpine Lodge maybe?). It's my favourite place in NZ and I go as often as I can, and I can only give my strongest recommendation to stay a night there. You won't find anywhere remarkable to stay between Mt Cook and Chch, it's mostly small towns such as Geraldine, Fairly and Timaru.

I would suggest staying in Twizel, then drive into Mt Cook in the morning on the way to CHC. It's a long drive from Te Anau and driving in in the morning, you might get better morning views of the mountain than in the late afternoon. Its another hour from Twizel to Mt Cook. If you have 4 hours in the morning to do Hooker Valley hike it is one of the highlights of our recent trip. Its almost flat all the way, but you get a gradual incline and the most fabulous panoramic views as reward and huge surprise at the end at Hooker Lake/Glacier.