You Do Need power plug adapter while traveling from Montserrat to Niue.

Here is Why You Need An Adapter

The outlet and plug type used in Niue 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

Niue

In Niue, outlets of type I are in use.

Type I