AKL, while it could be better, is a decent enough airport for international to domestic New Zealand. However, you state that the carriers are the same, so I assume you are flying Air New Zealand when arriving AKL? Or are you arriving on SQ? (This is how I read your post.) SQ does not operate domestic flights within New Zealand.
At any rate, any airline involved will not protect you if you miss the separate revenue ticket, whether more or less than MCT. The international and domestic terminals at AKL are not together, and require either a shuttle bus transfer that runs 3-4 times an hour, or a 15 minute outdoor walk. So, if you do book a shorter connection, still allow 60 minutes. If everything goes PERFECTLY and you aren't checking baggage, you'll get to the domestic gate in about 40 minutes. 60 minutes is theoretically workable without checked baggage if you can get your domestic boarding pass in advance and your incoming flight is not late, but there is a reason the MCT is 90 minutes, trust me. I've been through AKL airport I think four separate times at this point, I know it relatively well, and I've got two more passes through next month. The airline might put you on the next flight of the day, but sometimes some destinations (Dunedin/DUD among others) are only once daily out of AKL, Queenstown/ZQN services are daylight only, and many of the smaller towns are also not frequent. Only Wellington/WLG and Christchurch/CHC and a couple of other places are frequently served out of AKL.