Take your montain bikes on Sealink (Subritsky) ex Wynward Wharf in Auckland to Port Fitzroy on GBI. Stay at Fitzroy or nearby (Orama Holiday Park has selection of cabins and chalets - bit untidy, but clean - lovely; right on harbour's edge) and check out beautiful West coast - we hired a double ocean kayak and spent half day checking out islands (there are lots!) and remote beaches; rock formations: caves. Just perfect! Then a couple of days later, bike over steeeeeep hill (gravelled road makes it even harder!) east towards ocean beaches. These beaches too are lovely, but are white sand with rolling surf. We stayed at Palmer's beach (good self-contained chalet at expensive $175 NZD for the night), but you could choose Medlands, which is a bit south. Stay a night or two, then head via Claris (main airport here), Medlands then to Tryphena to meet boat out.
As an alternative, there is a forest track through DOC-administered land between Fitz and Whangaparapara, which is about half way between Fitz and Tryphena. We took the roads instead. Incidentially, weather was just perfect and we went late Nov (last week!) and there are next-to-no tourists thus virtually no traffic. During Xmas hols, population swells from 1500 to 15000!
Reason for boat (which is actually daily car/freight ferry) is that planes can't guarantee taking bikes. But you can hire them from a couple of places right beside Claris Airport. Boat takes nearly 5 hours to get to Tryphena then stops there for about 3 hours before heading to Fitzroy - another couple of hours); goes AKL>TRY>FITZ, but easier to go back to AKL from TRY (I think only 1 boat/wk from FITZ) - check www.sealink.co.nz. You can leave your car at Auckland Fishmarket right across road from Wynward Wharf for about $35/week.
A great 4 days!! And I lost weight too! with all the cycling, which is quite hard with the hilly terrain. But fantastic experience for my wife and I. Highly recommended. Next cycle trip for us is Rail Trail in Otago early next year!
The falls are the Kauri Falls, which are about two miles from Whangaparapara (bay of many mud flats). The photo does not really show them well as the falls are about 30 ft high.