Hi Paul, the 3/4 will be perfect for the streams up North, the #5 is perfect for most of the rest of NZ including Lake Tomarata, the 7/8 would be good if you do want to have a go for Kahawai or Snapper in the Salt. There is some fantastic Salt Water Fly fishing in the Warkworth area, Including Leigh Harbour, Omaha harbour, Sandspit and Takatu Point. I can also recomend the Goat Island marine reserve for anyone in the family who likes snorkeling, you will see many big Snapper, Blue Mau Mau and other fish if you swim here.
There are quite a few charter boats that go from Leigh or Sandspit, including at least one (I will try to remember his name for you) from Sandspit that does a fair bit of SWF. March is a great time of year down here, you will not need waders, wet wading in a pair of shorts over some poly long johns will be fine. The long Johns will offer some protection from both the sun and any sandflies you encounter.
All the best.
Mike
Here you go, the Sandspit boats website is here
http://www.sandspitfishing.co.nz/
Nick is a young skipper but he is a very good fly fisherman.