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

Here is Why You Need An Adapter

The outlet and plug type used in South Africa are C, M and N and in Tonga is I. This is how they look like.

Tonga

In Tonga, outlets of type I are in use.

Type I

South Africa

In South Africa, outlets of type C, M and N are in use.

Type C

Type M

Type N