You Do Need power plug adapter while traveling from Niger to Cook Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Cook Islands is I and in Niger are C, D, E and F. This is how they look like.

Niger

In Niger, outlets of type C, D, E and F are in use.

Type C

Type D

Type E

Type F

Cook Islands

In Cook Islands, outlets of type I are in use.

Type I