We are regular users of mixed gas here at TDNZ and have our own portable compressor and trimix blending system, although it is only in the last few years we have been able to say we do more nitrox and trimix dives than air dives. We have some excellent sites for deeper trimix dives, however we also suggest you check out our cave diving and wreck diving pages as well.
Nitrox is available in most large centres in NZ, but it is still considered the exception rather than the rule and it is generally only dive shops which offer nitrox, rather than charter operations. There are a few exceptions to this rule, but if you want to guarantee access to nitrox you will have to search around a bit. Northland Dive in the Bay of Islands, OceanBlue Charters in Tutukaka and the Lermontov Lodge are all operations we use regularly and all have nitrox available.
Trimix is virtually non-existent in NZ and getting a fill through a mainstream dive shop is practically impossible. The only options for trimix fills are through a few individuals/technical instructors who blend it mainly for their own diving use. Additionally, getting decompression gas fills can be complicated, due to some very odd dive tank legislation here in NZ.
Whilst getting fills can be difficult the effort is certainly worthwhile. At sites such as the Poor Knights and Bay of Islands we have numerous walls and deep reefs with easily accessible depths to 100m+. The visibility is generally excellent, there is lots of life to see and because much of the diving is on walls you have plenty to see and enjoy during your decompression stops.