When I was looking a year or so back, 4-pile moorings seemed like the best bet for long-term -- they were cheaper than either docks or dry storage and more secure than swing moorings.
But your mileage may vary.
A quick search found this:
Rates for Marsden Cove in Whangarei: Marsden Cove Marina - Whangarei, Northland : Rental
They don't appear to have swing or pile moorings. Opua Marina in the Bay of Islands does; for a 10-metre boat they vary in peak season from about $190 to $360, depending on heavy a block you want.
A swing mooring has the disadvantage that it's only as secure as your tackle; big storm could see you chafe through and come loose.
Opua also has a hardstand
, which appears to cost about $25 a day, plus the cost of haulout and launch.