Complex sentence: In 1990, the Cook Islands Netball Association had 15 clubs affiliated with it, all based in the southern islands group, including Ngatangia/Matavera, Avatiu/Nikao, Arorangi, Titikaveka, Takuvaine, Tupapa, Outer Islands.
Split into simpler sentences: In 1990, the organization had 15 clubs affiliated with it, with all the clubs based in the southern islands group. The clubs Ngatangia/Matavera, Avatiu/Nikao, Arorangi, Titikaveka, Takuvaine, Tupapa, Outer Islands.