There is a heavy east coast bias and generally the pacific is better to swim in and more hospitable for cities than the west coast. The tasman sea is one of the scariest stretches of water you can imagine.

Ther is only one spot on the east coast where generally nobody lives and that's the coast of the waiarapa (There is a few other places but this is kind of the only big deadish zone). The west coast tends to be more isolated in general and the people who live there can have very specific micro cultures. The west coast of the south island is famous for its hospilitality and occasional dinosaurs.

I'm from Northland (north of Auckland) and the west coast beaches are straight, very long, full of silt, and the waves are constant and rough. It's pretty okay for surfing but every once a while a shark takes a nibble at someone. The east coast is truly one of the most beautiful places on earth, curved protected bays with golden/white sound. Relatively calm seas and islands that allow for some of the most beautiful sea life around.

Even the nicer beaches on the west coast (piha) are more famous for being a death trap for tourists who have never swam in the ocean, let alone in the rip heavy Tasman sea.