You Do Need power plug adapter while traveling from Papua New Guinea to Namibia.

Here is Why You Need An Adapter

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

Papua New Guinea

In Papua New Guinea, outlets of type I are in use.

Type I

Namibia

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

Type D

Type M