You Do Need power plug adapter while traveling from Montserrat 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 Montserrat are A and B. This is how they look like.

Montserrat

In Montserrat, outlets of type A and B are in use.

Type A

Type B

Papua New Guinea

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

Type I