I've just got back from NZ - I was on a family holiday in the Coromandel area on the North Island. I wasn't kitted up to do any serious investigation but many interesting old mines which look as if they would be accessible if you could get to meet the right people.
The Talisman mine at Karangahake was an excellent walk armed with just a hand torch - the further reaches of the mine are gated but it does look as if people are exploring beyond. Lots of good surface remains as well. Seems to be controlled by the department of consevation but I hear that the local gold mining company also have an interest and may be looking to restart underground operations.
Talking to the guy at the mining museum (school of mines) in Thames it appears that there is not a very active mining history community, few books etc. He did say that he could arrange for one of the local mine historians to take me for a walk round the area - but sadly there was no one available that evening and we were due in Aukland the next day. I did have a brief walk in the woods around Thames and there are many open adits which look good to explore.
Plenty of potential!