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

Here is Why You Need An Adapter

The outlet and plug type used in Bhutan are C, 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

Bhutan

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

Type C

Type D

Type G