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

Here is Why You Need An Adapter

The outlet and plug type used in Mauritius are C 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

Mauritius

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

Type C

Type G