You Do Need power plug adapter while traveling from Niue to Sierra Leone.

Here is Why You Need An Adapter

The outlet and plug type used in Sierra Leone are D and G and in Niue is I. This is how they look like.

Niue

In Niue, outlets of type I are in use.

Type I

Sierra Leone

In Sierra Leone, outlets of type D and G are in use.

Type D

Type G