Visiting NZ - March vs April?

Just wondering what the pros and cons may be in visiting NZ (both North and
South Island) in March vs April, for someone who is not going specifically
to spend a lot of time on the beaches.

Of course, it depends on individual preferences, but I'd appreciate comments
from folks who live there, or who have visited there.

Some considerations that come to mind include:

-weather - temperature, rainfall
-are tourist attractions/services/points of interest open/available as much
in April as they would (presumbably) be in March?
-Is there much difference in rates for accommodation etc?
-Some people posted about sandflies in the southwest areas etc. Are these
better/worse in April/March? (By the way, just curious - are these similar
to what we call blackflies in North America?)
-Any other considerations?

Thanks
Ron B

(to respond directly, please delete the prot)