I wouldn't bother with Wellington Zoo if you are going to spend a year in Sydney - you have heaps of time to go to the zoo there.
The Little Blue Penguins in Oamaru are lovely - but there are not as many in Jan as earlier in the season. I haven't been to the Yellow Eyed penguins but apparently they are good too.
Your dh could walk up the bridle path - the path the first arrivals in Christchurch had to take from the harbour over the hill - it takes me around an hour, but if he's fit it wouldn't take him as long. You and the children could get the gondola up and have a snack with a 360 degree view of a real volcanic crater, then drive through the tunnel and pick him up in Lyttelton.
The Hector's dolphins are very small and cute and you can swim with them in summer, or just get a boat trip and look at them.
The seals in Kaikoura are worth a look - they are free, and best at high tide, but don't get close, they stink and will honk very loudly at anyone that gets near them.
In Wellington I absolutely love Te Papa, the cable car, the botanical gardens. Ds adores getting the train, but that's because we don't have commuter trains in the South Island, so maybe not so much of a treat for your guys.
I wouldn't do the Antartic centre in Christchurch - I would do Kelly Tarltons in Auckland instead.
I'm sure I'll think of more later.