You Do Need power plug adapter while traveling from Namibia to Nauru.

Here is Why You Need An Adapter

The outlet and plug type used in Nauru is I and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Nauru

In Nauru, outlets of type I are in use.

Type I