Kiwi accommodation generally caters fairly well for travellers with mobility issues, with most hostels, hotels and motels equipped with one or two wheelchair-accessible rooms. (B&Bs aren't required to have accessible rooms.) Many tourist attractions similarly provide wheelchair access, with wheelchairs often available. Most i-SITE visitor centres can advise on suitable attractions in the locality.
Tour operators with accessible vehicles operate from most major centres. Key cities are also serviced by 'kneeling' buses (buses that hydraulically stoop down to kerb level to allow easy access), and many taxi companies offer wheelchair-accessible vans. Large car-hire firms (Avis, Hertz etc) provide cars with hand controls at no extra charge (but advance notice is required). Air New Zealand is also very well equipped to accommodate travellers in wheelchairs.
Download Lonely Planet's free Accessible Travel guides from http://lptravel.to/AccessibleTravel.
Out and about, the DOC has been hard at work improving access to short walking trails (and some of the longer ones). Tracks that are wheelchair accessible are categorised as 'easy access short walks': the Cape Reinga Lighthouse Walk and Milford Foreshore Walk are two prime examples.
If cold-weather activity is more your thing, see Snow Sports NZ's page on adaptive winter sports: www.snowsports.co.nz/get-involved/adaptive-snow-sports.
Access4All (www.access4all.co.nz) Listings of accessible accommodation and activities around New Zealand.
Firstport (http://firstport.co.nz) Includes a high-level overview of transport in NZ, including mobility taxis and accessible public transport.
Mobility Parking (www.mobilityparking.org.nz) Apply for an overseas visitor mobility parking permit ($35 for 12 months) and have it posted to you before you even reach NZ.