Abel Tasman National Park
The Coast Track
Sandflies aren't really a problem in the Abel Tasman National Park - until you sit still.
They prefer shade to bright sun - so, seek cool shade and you may be driven insane by them. Use a natural repellant in a beeswax base. That'll stop the itching and prevent more bites (for a while).
Despite having the second scariest name in the Abel Tasman ("Mosquito Bay" is probably slightly worse), Sandfly Bay has no more sandflies than anywhere else in the park.