You Do Need power plug adapter while traveling from Guinea-Bissau to Niue.

Here is Why You Need An Adapter

The outlet and plug type used in Niue is I and in Guinea-Bissau is C. This is how they look like.

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C

Niue

In Niue, outlets of type I are in use.

Type I