The above 'March is a bad month' posters are west coasters - who for most of the year enjoy strong SW flows. March is right in the middle of their low season for wind.
In late summer the best bet in NZ is the NE coast of the North Island, high pressure systems and tropical cyclones bring E winds and good swell.
Start in Auckland, rent a car and travel north, there are 100's of great beaches, river estuaries and surf breaks that work well that time of year.
Wind stats from my old local here:
www.windfinder.com/windstatistics/mokihinau?utm_source=www.windfinder.com&utm_medium=web&utm_campaign=redirect Cheers
DK
PS - There is a full moon in March on Thursday the 5th, going on way to much experience of such things I'll bet there is a Strong E to SE wind from the 5th right thru the weekend of the 7th/8th in the Ruakaka/Mangawhai area. If I was planning a trip that is where/when I would go.