How can I make several polygons with alphaShape from a shapefile that contains multiple line objects?

I downloaded a shapefile with the coastline of New Zealand. I would like to check several coordinates to see if they are within the land of New Zealand. (I found out that I should use the inShape function.) Since the shapefile contains several islands/polygons, when I use alphaShape it makes one huge polygon which is a mess. How can I separate the different islands/polygons? Thank you for your help and answers!

Answer 1

[Matt J]

Use a smaller alpha radius.