Pete,
Bit of Motorsport happening in SI of NZ in January / February... 8/9/10 Jan Wigram Revival @ Ruapuna Ch Ch, F5000 series & others, 16/17 Jan NZV8's @ Teretonga Invercargill, 5/6/7 Feb Skope Classic Round 1 @ Ruapuna ChCh.
You could fly in from Brisbane /Queenstown & out ChCh/Brisbane ( Or Vice Versa ) if you want to save some drive time. Q/town is only about an hour away by road from Wanaka/Cromwell etc with all the winery tours etc plus Warbirds collection in Wanaka. In mid Jan you will be amongst the Mom & Dad family holiday bunch ( The younger New Years Eve mob will be back at work paying for their sins

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Invercargill is about 2 1/4 hrs south of Q/town, Te Anau..Milford is a day trip from either or you could overnite in Te Anau to make it more relaxed for a first timer. Chopper or Boat ride in Milford is a must... but not with a hangover!!! ( BTW, the info regarding chopper ride was from my daughter...you couldnt pay me enough to ride in one of those things.. which is a bit of a worry in NZ where all medivac is by chopper.. guess I will have to stay healthy

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Going North you can go up the east or west coast, plenty to see in either direction, I see Di loves fishing, plenty of opportunities for both fresh & salt water fishing boat rides.
If you want to ride/fly a Tiger Moth then Croydon A/C Co @ Mandeville about 10 miles from me will oblige & if your into De Haviland stuff its a must see... Morton will be Mortified & for once you will have seen something he has not!!
Lots more I cant think of, can you be a bit more specific on what stuff you & probably more important ,Di are interested in...........