You Do Need power plug adapter while traveling from Tonga to Dominica.

Here is Why You Need An Adapter

The outlet and plug type used in Dominica are D and G and in Tonga is I. This is how they look like.

Tonga

In Tonga, outlets of type I are in use.

Type I

Dominica

In Dominica, outlets of type D and G are in use.

Type D

Type G