There is a provision in the building act (third schedule) that exempts buildings up to 10 sq metres and one story high, if sighted at least the distance from the boundary that it is high, from needing a building consent. This is to allow for a garden shed to house a lawnmower and garden tools. The exemption has "provideds" that go with it and you will probably find that the use you are providing will do away with the exemptions because of the facilities you will require. You will require a letter from the local and regional councils before mpi will approve.
The requirement that may be a problem is this part in the exemptions
(iv) Does not exceed either 1 storey, or 10 square metres in
floor area, and does not contain sleeping accommodation or
sanitary facilities or facilities for the storage of potable
water:
You will need a sink and wash hand basin plus plumbing and drainage which will also require a building consent.
You may also require a consent for change of use and you may require a resurce consent if quarantine is not allowed in the zone you are in.