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

Here is Why You Need An Adapter

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

Swaziland

In Swaziland, outlets of type M are in use.

Type M

Papua New Guinea

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

Type I