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

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I and in Thailand are A, B, C and O. This is how they look like.

Thailand

In Thailand, outlets of type A, B, C and O are in use.

Type A

Type B

Type C

Type O

Tonga

In Tonga, outlets of type I are in use.

Type I