Queenstown I've spent time windsurfing in summer months and awesome but still need a 4/3 wetsuit with cold lake water.
With the shape of the lake it can be windy on one arm of the lake and glassy the other depending on wind front direction or summer thermals. Great spots like Glenorchy, one mile and Jardines.
Generally small gear
Queenstown is an expensive place and ski fields for winter, great mountain biking.
Wanaka you are closer to better ski fields Trebel Cone and Cardona. Apparently good windsurfing in summer.

^ Seventh wave wetsuit?
Agree, only racing in both. Second day in Dunedin blew hard and racing from yatch club with great vibe.
Chch locals talking up Lake Coleridge with good jumping on small gear

Hi Cald the bottom of the South Island is unreal, l normally sail there every summer not this year due to Covid. There is a Southern lakes Windriders Club and they have away trips to the south coast and let everyone know whats happening. Dunedin is also a great spot and a great Slalom Racing location.The lakes can be very strong wind and 4 m sails are not uncommon.The south coast has some very good wave sailing Colac Bay and Monkey Island also bump and jump sailing at Awarua Bay near Bluff. So many very good lakes with great people sailing them. I hope to get back next summer.

I spent 6 weeks on the South Island on my trip to NZ , mid Feb to late March, 2002. I've just been looking through my photos. On the lakes I used my F2 Axis 262 & 5.8m. I only had a 3mm summer suit and bought a 2mm vest to go under that as even Wellington was cold in a southerly in the summer.
I stayed in Christchurch and sailed at Lake Ellesmere (warm and shallow compared to the rest of the lakes). I drove to Lake Coleridge, 1 hour along the main road then another hour along a gravel track the last 20k. East side, about 1/2 way up. Looking on the map I think I went a different way back as it was quicker. Stunning place to sail. Wondering if the gravel track is still as bad as it was or if its been tarmacked more since.

Next was Dunedin and sailed on the sea at the end of the inlet, Aramoana. Walking down the beach I passed a sleeping bull sea lion, huge thing, 2.5m long. After a couple of hours I saw a big shadow under the next wave out so gybed and sailed to the nearest bit of beach. Scared the sh1t out of me to be honest. I noticed the sea lion had gone off the beach, I reckon it was him coming out for a surf. I didnt go back in!

Invercargill was extremely cold. I had a look at the nearest beach and it was cross on and wild, so found a more sheltered spot near the Bluff iirc. Still way too much for a 4m & my small K-Bay 75l custom. I called in at Colac Bay, but it was way too cold for me to go out (of the car). Its was the only place I saw another windsurfer on the South Island. They had full winter steamers on.

Te Anua was another session on a lake. The wind was westerly so nothing on most parts of the lake so drove north to where a side valley came into the lake and the wind was blowing down that. So has a session up and down the beach.

Next was Queenstown, the wind was from the south about 18 knots so sailed 1/2 way along that leg of the lake. Driving to Queenstown, a few miles away from where I sailed, the lake opened out and there was 10 knots wind.

Wanaka next. 1st day no wind in town by the lake but 5k just to the north of town the wind was funnelling down a valley so had a session there. Dublin Bay I think. After a while the wind changed direction a few degrees and I was left stuck with no wind, the wind line was 50m away and I had to swim to it. I sailed on Lake Hawea for the next 3 days.

Driving up the west coast from Haast the beaches were wild and deserted and the waves were dumping on the beach.

I guess when I was there people were working so thats why I didnt see anyone else, or they all knew better places to sail for the conditions on that day. With all the lake's different orientation and side valleys it must take a while to build up decent knowledge.

Christchurch & Dunedin being larger places had more things going on off the water. I think my favourite place overall was Wanaka.