Online it said Milford sound and west coast-- is that the entire west coast? Which specific areas?
Also, if we're going on a tour of Milford sound with cruise on the water for 2 hours- are there sandflies there too?

While Milford Sound, Fiordland and the West Coast (yes, the entire West Coast!) are generally the worst areas for sandflies, they can be found many places on the South Island, including Abel Tasman, Mount Cook, Glenorchy, etc.

They're not a problem when ON the water, but can be pretty fierce otherwise. Wearing long sleeves, long pants with long socks and of course insect repellent will all help.

Sandflies are slow - it helps to keep moving. If you're walking a track for instance, you usually won't notice them until you stop. Then all bets are off!

IME they're worse when the weather is overcast and humid, not as bad when it's sunny and warm, but take that with a grain of salt.

This map illustrates what Melnq8 is saying about the distribution of sandflies:

https://teara.govt.nz/en/map/14737/sandfly-nuisance-map

That was certainly our experience when we hiked on the South Island last month; there was one morning on the Milford Track when my arm was pretty much black with them, despite having applied repellant, but I can't recollect being bitten once we got moving.

Thanks for the info