Frank, this is a start, there are a few places to see Kauri I will check out the places for you.
Heather

Here is a link that might be useful: Gardens to visit.

There are so many I don't know where to start! Will put my thinking cap on for the best ones.... Philippa

Waipua forest for Kauri trees...Head to whangarei take left hand road to dargiville..the kauri coast....pass through dargarville and between there and the Hokianga harbour is the Waipua forest...the place where Tane Mahutu abides there is also a kauri museum just past Maungataroto ...well sign posted all of it.

I think the most famous arboretum type place is the Pukeiti (hope I spelt that right) Garden Trust which is somewhere around New Plymouth. Big collection of Rhododendrons. The native nikau palm grows luxuriantly in that district too. Apart from that New Zealand doesn't have many arboreta - but its very easy to see good specimens of the native trees in the bush. The trouble is identifying them, if you aren't familiar with our flora. J.T. Salmon's guide to the trees is a standard work with good photographs. That's what I used to learn the trees.

As others have indicated, northland (the peninsula north of Auckland) is the best place to see kauris, (and also a number of other subtropical trees like Beilschmiedia taraire). Head for Waipoua Forest. A newly-tarmac-ed state highway runs through it with tall kauris on either side plus a wide diversity of other trees, cabbage trees and tree-ferns. Very photogenic, but often can be rainy. The Department of Conservation has lodges right in the forest you can stay in, something I would like to do.

Also just on the western fringes of Auckland there is the Scenic Drive through the Waitakere Ranges. Visitors from overseas are often reluctant to do this trip, because they are convinced that you have to travel to remote corners of the country to see good native flora, but this isn't the case at all. The Waitakeres hold a diverse flora, including kauris, though not as big as the giants in Waipoua. If you go, its worth dropping in to Landsendt in Parker Road, Oratia, a subtropical nursery with good display grounds - mature Parajubaeas, Howeas, Archonotophoenix, Caryota gigas, gigantic pair of Ceroxylon alpinum, all manner of bananas and bromeliads. There's also a bamboo nursery nearby (could be in the same road) that is by appointment only; Isaachson's. I think they have some of the tropical Gigantochloas, Dendrocalamus and Bambusa species as well as the larger Phyllostachys species, if you are interested in bamboo.

Auckland has a botanic garden right off Highway 1 south of the city centre, but its recently established and not that amazing, (yet, anyway).